In YuYu Hakusho, Kurama already bears some resemblance to Himura Kenshin, but for a couple episodes of the Tournament arc, they went out of their way to make it explicit by giving him a cross-shaped scar. Megumi Ogata voiced Kenshin in the Drama CDs.
The anime adaptation of Lucky Star is, arguably, half-built on such heavy references. The most emblematic, however, is from episode sixteen — when Konata Izumi cosplays at her cafe as Haruhi Suzumiya. Both characters are played by Aya Hirano, who effortlessly slips back and forth into her Haruhi voice to take orders in the cafe. (In the English dub version Wendee Lee tries this but can't seem to sound like she did back in 2007, this same thing happened with Michelle Ruff as Nagato, although she sounds like herself again in the preview, the only character to sound like himself was Crispin Freeman's Kyon, probably due to the fact the dubbing was lazy in volume 4, compared to the other volumes.)
Konata had previously attended a concert (Suzumiya Haruhi no Gekisou) with Aya Hirano (technically voicing herself with a Suzumiya HaruhiImage Song recording) as one of the stars.
In episode twenty-three, Konata buys a character Image Song for Galaxy Angel Rune's Kahlua, who is voiced by (you guessed it) Aya Hirano, due to feeling "somehow connected" to it. She then convinces Kagami to buy a Powerpuff Girls Z album, where Kagami's voice actress Emiri Katou voices Hyper Blossom.
A few scenes in episode seven consist of Konata using ultra-polite Japanese and the phrase "A girl should be elegant", traits of another character once played by Aya Hirano: Lumiere from Kiddy Grade.
Even earlier, more obscure Hirano roles aren't immune: Konata's musings about "that show where a guy's pets come back to life as girls" are a reference to Angel Tales, where Hirano played Momo The Monkey.
This was already there in the manga—should it count?
Manga volume 5 contains a bonus strip in which Konata (and Hiyori) see Aya Hirano in the coffee shop they're at, eating a chocolate coronet.
Daisuke Ono as himself and Akira Kogami in Lucky Star episode 20 lampshade their previous roles in the Kyo Ani adaptation of AIR: Ono Daisuke as Yukito Kunisaki/the crow Sora and Hiromi Konno as Potato, Kano Kirishima's dog.
Akira: You're not a crow today.
Daisuke: You're not a dog today.
Akira: Picco picco.
Sadly, this joke is lost in the dub, mainly because AIR is dubbed by FUNimation. And a general rule of thumb, you really can't use VA from California to do Texan anime dubs due to...legal reasons.
If anyone is curious, Sora and Potato are voiced by Vic Mignogna and Tiffany Grant respectively in the English dub AIR (although the earlier does voice some minor characters in the Lucky Star dub).
All of the SOS Brigade members' voice actors have appeared in Lucky Star at one point or the other. Aside from the obvious Aya Hirano (Konata/Haruhi), Minori Chihara (Minami/Nagato), we have Daisuke Ono (Koizumi) as himself in one Lucky Channel, Tomokazu Sugita as mentioned above, and Yuko Goto (Mikuru Asahina) as a yakuza-like muscular version of her real life self. The dub actors for the Brigade are also present in the Lucky Star dub.
We should also note that, in the English dub, Konata is voiced by Wendee Lee, Haruhi's English voice actor. This was likely a conscious casting decision (though they were lucky she had enough range to allow it).
As was Sam Riegel — who voiced Taniguchi — for Minoru Shiraishi. In fact, his first line in the second episode is the start of "Ore no Wasuremono". Not the first time he sings it, either... it eventually got a full version on his ending theme CD after Lucky Star ended.
Don't forget: anytime any guest VAs turn up in Lucky Star, it's almost certain that the next episode preview will be voice-overed by whatever character they're famous for. Example: Kyon and Yuki previewing episode 17, or the cast of Keroro Gunsou previewing episode 13, after their VAs turned up in the anime, In the English dub of that same episode, Vic Mignogna apparently reprised his role as Keroro (and is the only ADV cast member from the show to do so; he couldn't keep his role in the Funimation dub).
There is an inversion of this in Haruhi-chan; in addition to the show itself having a more Lucky Star-like art style (though by no means identical), one particular episode has Haruhi carry out a good number of Konata's mannerisms including a shot that mirrors Lucky Star's teaser shot exactly. Haruhi's voice also seems to have been modified a tad to sound a bit more like Konata.
Episode 5 of Pokémon contains one during a rather elaborate Shout-Out to Episode 48 of Fushigi Yuugi: In FY, during the fight between Tamahome and Suboshi, Suboshi ends up getting killed by his own weapon when he finds himself pinned down by the ghosts of Tamahome's brothers and sisters (whom Suboshi had killed) and unable to get out of the way. In Pokemon, just as Satoshi (Ash) is about to order Pikachu to deliver the finishing blow to Takeshi's (Brock's) Onix, he imagines himself being held back from giving the order by Takeshi's many younger siblings (though it turns out they really are holding Satoshi back). What makes this an Actor Allusion is that in the Japanese version of their respective shows, Takeshi and Suboshi are both voiced by seiyuu Yuji Ueda.
The dub gets to pull this when Maylene is introduced. Jessie comments that it's "A meeting of old and new!", likely referencing that Maylene's voice actress was Jessie's 4Kids VA — Rachael Lillis. A Hoenn episode also had a very sneaky one for the same actress, titled "A Different Kind of Misty". Guess who voiced one of the characters of the day?
Here's one more, the actress who originally played Ash on Pokémon (Veronica Taylor) plays the role of Carly on Yu-Gi-Oh 5Ds. Now here's where it gets freaky: turns out that both characters (Ash and Carly) have the same aspect to their personality (getting into tight spots) and not only that, both have a friend named Misty. You can see information on the four using the following four links Ash Ketchum, Misty (Pokémon), Carly and Misty Tredwell Misty Tredwell being from 5D's of course)
In the Japanese version, Saori (Solidad), the pink-haired Pokemon Coordinator, is voiced by Rie Tanaka, who was the voice of LacusClyne, the pink-haired Coordinator.
The episode where Jessie tries to train to become a nurse is a reference to her voice actress Megumi Hayashibara, who was also training to be a nurse in the early days of her career.
In "Air Time", the role of the hero's Charizard is given to James. In the Japanese version, Shinichiro Miki plays both.
Episode 9 of Keroro Gunsou contains an Homage to episode 24 of Neon Genesis Evangelion where Mutsumi descends into the Keroro Platoon's base in the same way Kaworu (who was also voiced by Akira Ishida) descended into the NERV headquarters, complete with the same background music.
Episodes 14 and 15 contain one Dragon Ball Z shout out each in the dub (but not in the original Japanese version), with Giroro doing the referencing.
In the dub of episode 24, Lavie's TV show is called "Gunbringer Girl". Mois thinks it sounds familiar — her voice actress, Carrie Savage, having had a small role in Gunslinger Girl. Again, there is no equivalent reference in the Japanese version.
Asuma Sarutobi from Naruto often says "yare yare daze" and "ora ora" (moreso in the PS2 Naruto games). While these phrases are used often in other anime, in this case it's a reference to Asuma's seiyuu Jurota Kosugi's previous role as Jotaro Kujo in the OVA adaptation of JoJo's Bizarre Adventure.
When Sunrise changed the last names of nearly all Mai-HiME characters for Mai-Otome, Haruka Suzushiro became Haruka Armitage, a reference to one of Ryōka Yuzuki's previous roles — Naomi Armitage from Armitage III.
Another example is Midori, a character who likes to tell people that she's "just 17 years old", a reference to a joke by fellow seiyuu Kikuko Inoue (who voices Yukariko in both series).
In the first episode of Fushigi Yuugi, Miaka gives an In the Name of the Moon speech to a group of street thugs attacking her: Miaka's voice actress, Araki Kae, played Sailor Moon for several episodes while her regular voice actress was ill, and also played Chibiusa on the same series.
The English dubbing for Sailor Moon was done in Canada with Canadian voice actors.
Yuuichi, in the Kyoto Animation remake of Kanon, is played by Sugita Tomokazu. In one episode, he does a Deadpan Snarker narration monologue exactly in character as Kyon, Sugita Tomokazu's character from Suzumiya Haruhi no Yuuutsu. It doesn't turn out to be just narration, though — Kaori can hear him say it, and she's not amused. Again, just like Kyon. The scene is actually in the original Visual Novel though, but nobody really care.
In an episode of the anime, some schoolchildren point out that Hitsugaya Toshirou, disguised as a human, is a bit short, prompting him to shout, "Who's a super midget?" in an obvious reference to the role of Romi Park, his seiyuu, as the height-hypersensitive Edward Elric in Fullmetal Alchemist.
In an episode of the English dub, Uryuu ended up expressing frustration at Pesche, unhappy that Pesche was comparing him to various insects. Ironic, considering one of Derek Stephen Prince's other voice acting roles, Shino Aburame.
Same with Naoto Shirogane from Persona 4. They left the line for the anime too.
A similar gag occurs in the dub of Kekkaishi, with Tokine saying, much to Yoshimori's chagrin, that she isn't interested in guys who are significantly shorter than her.
Another episode has Madarao calling Yoshimori's actions "petty and small". This, of course, causes an outraged uproar.
Parodied in Toaru Majutsu No Index-tan, where Index copies the joke because JC Staff produced both shows, even encouraging the audience to purchase Shana DVDs if "they don't get it."
Hayate the Combat Butler has a large number of these; notably, the Puppet Butler with a distorted voice uses the Geass on Nagi. He no longer speaks with a distorted voice, and the VA is clearly Jun Fukuyama, the VA of Code Geass' Lelouch Lamperouge. He also resemble Watanuki from XX Xholic, another of Jun's roles.
In episode 10 of season 1, an intermission artwork shows Nagi dress up as Louise from Zero no Tsukaima.
Before this we saw Nagi's triple allusion attack on Sakuya as Bell, Shana, and Alphonse respectively.
And let's not forget the psychoticallyHot-Blooded redhead Combat Butler voiced by the same guy who did the voice of Gai in GaoGaiGar who uses pretty much all of GaoGaiGar's signature attacks, up to and including the Goldion Hammer. Appearance-wise, he looks like Viral with Gai's long hair.
Another one appears early on in the second season: Hayate, Nagi, Yukiji, and Wataru are in the dungeon with Sonia, and then at some point, the ghost of the priest Linn Regiostar tells Hayate to "Image it."
Even the Hayate manga is able to sneak in an actor allusion. In chapter 148, there's a parody of the class roster picture from Mahou Sensei Negima!. In the roster, Hayate is seen to have the same student number as Kaede from Negima. This is no coincidence, as Hayate and Kaede are both voiced by Ryoko Shiraishi.
In episode 35 Maria cosplays as Suigintou from Rozen Maiden. It goes without saying they are both voiced by Tanaka Rie.
The animated video made as an intro to "It's a Rumic World" (an exhibition of artwork by Rumiko Takahashi) has Inuyasha noting that Ranma and him have the same voice, because they also have the same seiyuu. Extra funny considering they also have the same English voice actors (well only seasons 4-7 for Ranma).
This is only one of the most subtle instances. The dub also includes someone claiming that Leo has a bigger ego than Chris Patton, who plays Hajime; another episode has Hajime repeatedly accusing Satsuki of sleeping with Vic Mignogna. And then there's the episode in which, after a ghost's curse puts Satsuki's father (played by Illich Guardiola) in the hospital, the characters refer to him as "Illich" for the rest of the episode.
Excel♥Saga has one episode which parodies anime and animation in general, and features its protagonist Excel, voiced by Kotono Mitsuishi, doing a takeoff of Sailor Moon, whom she also voiced. Some of the people watching her make fun of it, causing her to angrily spout a line like "I made good money doing this!".
In RahXephon, Haruka introduces herself to Ayato as a "champion of justice". The phrase and voice actress both were in Sailor Moon. (She voices Ami)
Of all things, the 4Kids dub of One Piece manages to get one in during the Drum Island arc. Dr. Kureha insists to be called Dr. K, a reference to the main villain of Cubix: Robots For Everyone. Both were voiced by the late Maddie Blaustein.
Jail Scaglietti shares some eerily similar traits to Scythe Master from the visual novel Phantom of Inferno. Ken Narita notably voices both characters.
Sora Wo Kakeru Shoujo has a character (sentient AI, same thing) voiced by Jun Fukuyama pulling everything from his time as Lelouch to create a madman in a robot body. The manga practically confirms this as he says "if I was human, I'd look like this" with a shot of Lelouch.
Earth Defender Mao Chan includes a character (Kagome Mishima) who looks identical to Ken Akamatsu's previous and better-known series Love Hina's Naru Narusegawa; another character (Sylvia) is voiced by Naru's VA Yui Horie. At one point, the characters visit a hot springs resort which is clearly the one from Love Hina - they even describe it as "the hot-springs which magically makes people get in to Tokyo U!" It's inhabited by another character looking identical to Naru, and when Kagome introduces herself to the Naru-alike and asks her name, another character says "No! Don't answer that!" In the English dub, Naru and Kagome have the same English voice-actress.
In the Gag Dub of Shin Chan Hiro and Mitzi have a faux-epic newspaper-sword fight and Hiro calls one of his attacks "Sword of the Darkness Flame", which was an attack of Hiei from YuYu Hakusho, both voiced by Chuck Huber.
Darker Than Black has one with April which is a combination of this, Captain Ersatz, and Expy. The character's Japanese voice actress dubs a lot of Halle Berry's roles, including that of Storm in X-Men. The character in the anime is a black woman with weather-control powers.
Hei's voice actor also voiced Tenma, and in the Time Skip between the two seasons, Hei mirrors him in going from clean-cut Mr. Fanservice to sexiest hobo ever.
In episode 3 of Rei, Rena had a Marimite parody moment with her being Yumi and Miyo being Sachiko. Miyo and Sachiko share a seiyuu.
Pani Poni Dash! builds an entire scene on this trope: the Alien Captain is played by Mugihito, who voiced Jean-Luc Picard in the Japanese dub. So given the nature of the series, it was inevitable that the aliens would become expies of the NextGen crew at some point.
The Legends of the True Savior movie/OVA series based on Fist of the North Star feature quite a few voice actors from the 80's TV series, even though most of the established main characters are played by new actors. One intentional example occurs in The Legend of Kenshiro, the prequel movie, which features Kenshiro befriending a married couple played by Keiichi Nanba and Miina Tominaga, who were the voice of the adult Bat and Lin respectively in the original TV series. Kenshiro later meets Shin's former sensei, Fugen, who is voiced by Takeshi Aono, the original voice of Rihaku.
A slightly complex example, and probably not intentional, but the fans have latched onto it anyway. In Tsubasa Reservoir Chronicle, Fai is voiced by Daisuke Namikawa. In the drama CDs, Namikawa also voices Fai's twin brother Yuui, who happens to be an Italian chef. Fast forward a couple years, and Namikawa is voicing both North and South Italy in Axis Powers Hetalia, who have an obsession with pasta. Cue Tsubasa fans writing fanfics with Fai and Yuui calling for PASTAAAAA~ at the drop of a hat.
This gets a possibly intentional canon reference at the very end of the fourth CD, in which Fai excitedly requests PASTAAAAA~ for dinner. Kurogane has a psychotic breakdown promptly thereafter.
In one episode of Gintama, Otae tries to goad Hijikata into saying one of Date Masamune's signature Engrish phrases ("LET'S PARTY!!") and straight up asks him if he's excited about the second season. Hijikata and Masamune are both voiced by Kazuya Nakai.
The manga makes another one later where Hijikata rides in on a nearly naked Kondo (who has motorcycle handlebars taped to his head like Masamune's horse in the anime) yelling "Here we go! Okay! Let's Party!"
A slightly (probably unintentional one) good example would be the episode where Sadaharu goes insane. They go to the baseball stadium to set up the two Miko sisters' trap and it's Mone's turn to catch the ball. She can't catch it but Gintoki dives and says EXACTLY "Make it. Make it. Make it! WHILE trying to catch the ball. Both Kyon and Gintoki are voiced by Tomokazu Sugita and it's interesting to note that Haruhi Suzumiya and Gintama aired almost at the same time only two days apart.
On at least two different occasions, Okita has said "Mind if I beat you? Can't hear your answer!", which is the Catch Phrase of Ryutaros from Kamen Rider Den-O, both voiced by Kenichi Suzumura. In one instance, Okita's eyes flashed purple, which is what happens when Ryutaros possesses people.
Gundam 00 also does this in S2 by revealing that Ribbons Almark was the first Gundam Meister having been the one to operate the 0 gundam his voice actor was the same as Amuro Ray from first gundam while the 0 gundam was based on the rx-78-2 gundam from that series the in-joke is lost in the US dub.
Akira Kamiya, the voice of Ryo Saeba in City Hunter and Angel Heart, was also the original voice of Kenshiro in Hokuto no Ken. Episode 42 of Angel Heart centers around Ryo dating a popular anchorwoman. When Ryo meets the anchorwoman's boss, he finds out that the man is an old comrade-in-arms named Sanemichi Bando. Mr. Bando is voiced by none other than Kenji Utsumi, the voice of Raoh (Kenshiro's rival) in Hokuto no Ken.
In Kiss X Sis, one episode featured Keita, Ako, and Riko going to Akihabara. For some reason, Ako dons cat ears, is called "Ako-nyan", and momentarily pauses in front of a music store with five mannequins, who sported 2 guitars, a bass, a keyboard, and a drum set. Naturally, these are all unsubtle nods to seiyuu Ayana Taketatsu voicing Azusa Nakano in K-On!
It's also a common on 4chan to make a "Same VA thread" where you post a picture of one character they play, and a quote from another role they played, often trying to find the most different pair of characters as possible. Really good ones can derail the rest of the thread.
Nico Nico Douga is rife of MADs or fake ED themes containing allusions to seiyuu's other works. For example, the Code Geass version of the 1st Durarara ending had Shinra's place taken by Lelouch, and both were voiced by Jun Fukuyama.
In one episode of Full Metal Panic? Fumoffu, Kaname muses that she'd like to install a device in Sousuke's brain that would let her zap him whenever he did something embarassing. Kaname's voice actress, Satsuki Yukino, also voiced Kagome from Inuyasha, who did much the same to the titular character via his Restraining Bolt necklace. The allusion is lampshaded when Kyouko protests that Sousuke isn't a dog.
The next-to-last scene of Ep. 12 contains the following exchange, occurring when the screen is dark: "Hey Luci, good job, that was fun, huh?" "Um, yeah, but you're not done yet. You still have another line, Chris Patton!" "Oh, wait. There's still more?" "Hello... Turn the page, goofball!" (references to Luci Christian, who plays Kaname Chidori, and Chris Patton, who plays Sousuke Sagara).
In Infinite Stratos, Ichika's duel against Rin has him posing and charging similar to Gundam Unicorn, a reference to the VA's role as Barnagher Links.
In Hidamari Sketch, Miyako cuts an apple into the vague shape of an animal head while Yuno is sleeping, but when Yuno brings it up to her, she claims it must have come from an alternate dimension. Yuno jokingly suggests that a fairy crafted it. Kaori Mizuhashi, Miyako's seiyuu, has voiced at least three fairies: Navi, Ciela, and, in the anime world, Pepper.
In the Smile Pretty Cure episode where Miyuki and Candy switch bodies, Akaoni sneezes Majorina's ring away. This spoofs a Garfield and Friends quickie in which Binky the Clown sneezes dust that came from a diamond away. The same person who voiced Akaoni, Iwasaki Hiroshi, did the voice of Binky the Clown in the Japanese version of Garfield and Friends that aired on Cartoon Network.