Voice acting in general is prone to this. While voice actors are generally freelancers, a lot of studios and casting directors tend to have their own little talent pool, though there is some overlap when it comes to popular and in-demand talent. For instance, while all three studios use Los Angeles-based actors, it's easy to distinguish a dub by New Generation Pictures from one by Bang Zoom! Entertainment or PCB Productions.
Actors in Joss Whedon projects have a tendency to reappear in his future projects. There's a handy table on Whedon's page on The Other Wiki showing which of his posse appeared in which of his projects.
And to keep the pattern going, Whedon's feature film The Cabin in the Woods stars Amy Acker (again) and Fran Kranz (from Dollhouse) and features Tom Lenk (from Buffy), while Alexis Denisof (again) and Enver Gjokaj (from Dollhouse) show up in The Avengers. Jeremy Renner (Hawkeye in Avengers) previously had a bit part in Angel, and Chris Hemsworth (Thor in Avengers) was one of the leads in Cabin.
Besides Dushku, Glau and Denisof, there are four less-well-known actors (Jonathan Woodward, Jeff Ricketts, Bob Fimiani, Carlos Jacott) who have appeared in three of Whedon's four TV shows.
This has now started to carry over to shows featuring former alumni who had worked on Buffy/Angel, as an episode of Grimm produced by David Greenwalt had Amy Acker (again!) as a guest star and will have Alexis Denisof as a recurring character. And Tim Minear's short lived show Drive on FOX starred Nathan Fillion.
In the season 1 DVD of Dollhouse, there's a whole behind the scenes look at all the cast and crew who return to work with Joss for the show. He's got a production posse that doesn't just act, but also write, produce, shoot, etc.
In one further unusual take on this trope, an episode of Dollhouse was directed by John Cassaday, the artist from Whedon's run on Astonishing X-Men.
Actors from Joss productions often appear together even without Joss himself being on scene. In Castle, star Nathan Fillion has often appeared opposite Firefly alums, culminating in "Headhunters", where even the network was promoting the Adam Baldwin/Nathan Fillion reunion. Less well-known is the fact that the Season 2 executive producer was Jose Molina... himself a Buffy and Firefly alum.
Much Ado About Nothing (2012) marks the reunion for so many Joss alums that it's pointless to count them. And the movie was filmed at Joss's house.
From Angel, Alexis Denisof and Amy Acker play love interests Benedick and Beatrice (no doubt delighting the fans who wished to see them together on the show).
From Buffy, Tom Lenk appears as Verges. Riki Lindhome was a guest on the show before appearing in Much Ado as Conrade. Also, Anthony Head was the original choice to play Leonato.
Coming from Firefly are Nathan Fillion as Dogberry and Sean Maher as Don John.
Dollhouse had Fran Kranz, who plays Claudio here, and Reed Diamond, who plays Don Pedro.
The pilot episode of Agents of S.H.I.E.L.D. had appearances by J. August Richards (Gunn from Angel) and Ron Glass (Shepherd Book from Firefly); both played the same characters in later episodes of the season. Late in the first season, Amy Acker (YET AGAIN) guest starred as "The Cellist", a person Coulson referred to in The Avengers.
Similarly, Power Rangers also has a lot of recurring writers, producers and actors. Back in the Saban days, they also did both other Toku shows and anime dubbing, so there was (by necessity) a lot of crossover between Power Rangers, VR Troopers and even Digimon (as Saban dubbed the anime).
Tim Burton: Get me Johnny Depp and my wife on the phone!
Casting Agent: I can't ever not do that.
[Casting agent flips open a phone with two giant buttons labeled "Johnny Depp" and "Helena Bonham-Carter"]
Helena lampshaded this by posting this picture on her Facebook page.
Any given Star Trek TV series is likely to share a significant portion of its crew with any other Star Trek series. Trek directors are especially likely to have started out as Trek actors. They also have a pool of actors that they like to pull from for recurring or one-shot characters, such that there are five actors who have played seven or more different Trek characters: Vaughn Armstrong (12 characters! 13, if you include that Mirror Universe character), Thomas Kopache (7), Jeffrey Combs (9), J.G. Hertzler (8), and Randy Oglesby (7). Joseph Ruskin's only got six, but he's got the distinction of having appeared in five Star Treks.
Relatedly, a pizza trophy for the late Majel Barrett, the First Lady of Star Trek, who was involved in every version of the franchise from its inception in 1966 to her passing in 2009: TOS Nurse Chapel and pilot episode's Number One; TAS Lt. M'ress; TNG & DS9 Lwaxana Troi; TNG, DS9, VOY, ENT, and ST2K9 Computer Voice. (This leaves off Star Trek: Discovery, Star Trek: Picard and the forthcoming Lower Decks and Michelle YeohSpin-Off.)
Star Trek: The Animated Series had a low budget that led to it using the main actors as guest actors, so James Doohan and the aforementioned Majel Barrett voiced almost every guest character across the series' run.
And it happened again on 24. Former Star Trek: Enterprise showrunner Manny Coto joined their staff for Season 6; then Brannon Braga came onboard for Season 7. That same year they had Trip Tucker, Dr. Phlox and Worf's brother in various one- or two-shot guest roles (hapless dock guard, hapless scientist kidnapped by Tony and Sangalan tyrant General Juma, respectively). At least for Season 8 they branched out to some hotshotpilots...
And more often than not, Rob Tapert (Lucy Lawless's husband) will handle producing duties.
Josh Becker, who grew up with Sam, Bruce and co. and made Super 8 Films with them in their teens worked on The Evil Dead Trilogy and Xena and has used the group in some capacity in all but one of his own Indie Films.
Otto Preminger had poor working relationships with many of his stars (including Linda Darnell, whom he directed in four films), but he still had a few favorite associates:
Laura stars Gene Tierney and Dana Andrews became Preminger's favorite actors to cast at 20th Century Fox. Tierney was Preminger's first choice for Lady Windermere in The Fan, but pregnancy prevented her from taking the part. In the 1960s, Preminger brought them back for a minor role in one film each after their lives had deteriorated, Tierney in Advise & Consent and Andrews in In Harm's Way. Both those last two films also featured Burgess Meredith, who appeared in four other Preminger films.
Character actor Gregory Ratoff was a good friend of Preminger from his earliest year at Fox, and appeared in three films directed by Preminger; Preminger also played a small part in Where Do We Go from Here?, which was directed by Ratoff.
Doro Merande played small parts in four Preminger films, and was also in several Broadway flops Preminger directed.
More closely associated with Preminger than any actor was Saul Bass, who designed the promotional materials and title sequences for most of Preminger's independent productions, including the Broadway plays This is Goggle and Critic's Choice.
Max Slater, a friend of Preminger's since his Vienna years, supervised the dialogue on most of Preminger's later films. He was born Maximilian Schulz, but he changed his last name to that of a character he liked in The Moon is Blue.
Ben Hecht did screenplays or rewrites for four Preminger films.
The cinematography of Preminger's films of the late 1940s through the 1960s was usually but not invariably by either Leon Shamroy or Sam Leavitt.
Directors Zal Batmanglij, Mike Cahill and actress Brit Marling are always working together, from their first film to their most recent work, The OA.
EXILE Tribe has a huge group of singers, dancers and actors under it's wings and likes to throw them together in productions, whether they're in-house or not. One of their biggest hits, High&Low, seemingly got every one in their staff together in the same show. However there are numerous amounts of shows where multiple EXILE members (the ones with good acting ability) show up in shows and movies together. Example shows/movies are are Sugarless, Wild Heroes, JAM, GTO Remake, Prince Of Legend, and Rokudenashi Blues.
In both television and film, Rian Johnson collaborates with cinematographer Steve Yedlin, editor Bob Ducsay, and actors Joseph Gordon-Levitt and Noah Segan. Johnson and writer-producer Moira Walley-Beckett also worked closely together on several episodes of Breaking Bad, to the point that Walley-Beckett specifically asked for Johnson to direct 'Ozymandias'; Segan also has an uncredited role as a fireman in that episode.