Homestar Runner has nine main characters voiced by Matt Chapman. Nearly every conversion in the show is an example of this trope, since there are only three main characters not voiced by him (Marzipan, who is the only girl, Pom Pom, who is The Unintelligible anyway, with Word of God saying his "voice" is a clip of Matt's brother Mike blowing bubbles in milk, and The Poopsmith, who has made a vow of silence. However, in order to celebrate Strong Bad checking his 200th email, the Poopsmith finally broke his vow of silence to sing the intro song. Here he is voiced by John Linnell of They Might Be Giants. As of his subsequent appearance, he's gone back to being silent, though.) The ultimate example is in One Two, One Two, which has Matt singing/talking to himself in six different voices at once. The only time that this isn't the case is if circumstances render it too difficult for Matt to do a voice; for instance, his brother has had to voice Strong Sad at least once, as Matt had strained his vocal cords.
Matt also voices nearly all of the secondary characters as well, including the cast of show-within-the-show Cheat Commandos, alternate Anime and "old-timey" versions of the main cast, and the lead vocals for Fake Bands Limozeen and Taranchula, the former in an impressive '80s-metal falsetto and the latter in a deep death-metal growl. Matt could give Mel Blanc a run for his money.
Heck, it'd be easier to list who ISN'T voiced by Matt.
It's all the more impressive when one of his characters starts doing impressions of another character.
Talking to Himself also occurs in-universe: The "Teen Girl Squad" sketches are created and completely voiced by Strong Bad, and the "Powered by the Cheat" animations are all voiced by the The Cheat, imitating the regular characters' voices (these imitations are voiced by Mike Chapman, doing his darnedest to sound like his brother).
Because many webtoons are a one-person operation, they tend to have only one voice actor: Matt Wilson does all the voices in Bonus Stage and Ed Atlin does all the characters in Space Tree, although both did when get another voice actress, Kagome Higurashi (not to be confused with the other Kagome Higurashi), who ended up voicing the female characters that were previously just pitch-shifted.
Ed also continues to voice all his characters in his series "Moonface".
The characters in Retarded Animal Babies are all voiced by creator David Lovelace, but tuned to different pitches. In one episode, the exact pitch levels he uses for each character are even revealed in the credits.
While other people play minor characters, all three main characters in Park Bench are played by Anthony Mercer.
The antagonism between The Leet World's Jerk with a Heart of Gold terrorist leader Cortez and counter-terrorist Team Dad Westheimer is made all the more interesting by the fact that they are both voiced by Eddie, who also voices the Camp Gay terrorist Montrose. Fellow crew member Daniel voices both hard-drinking fratboy Chet and the creepy Producer.
J.I.M., creator of Neurotically Yours, voices every male character on the show regardless of species. It is obvious this trope would come up at one point or another.
Several characters on Happy Tree Friends share voice actors. Cub, Giggles and Petunia (whose VA was replaced twice); Splendid and Lumpy (VA replaced once); Pop and Flippy. Lifty and Shifty also shared a voice actor until their VA left the show and was replaced by Kenn Navarro, also the VA for Cuddles. More recently, Pop and Flippy's VA left the show as well; while said VA is sampled for Pop and Flippy's evil side, Kenn now voices Flippy's good side.
Animator Brad Neely voices (almost) all of the characters in his cartoons, notably the eponymous Frank and Steve Smith of The Professor Brothers and Cox and Combes of the viral "Washington Rap." Just in these two examples, Neely has not only had lengthy talks with himself, but has duet-rapped.
The first episodes of A Day With Bowser Jr were mainly voiced by the same guy - Daniel, the animator. In fact, initially the credits would constantly read: "Daniel, Daniel, Daniel...."
Two of the main staff members from Rooster Teeth, Burnie Burns and Matt Hullum, voice multiple main characters from Red vs. Blue. In fact, Burnie made a point of voicing all of the Alpha AI fragments prior to season 9 (except for Delta, voiced by Mark Bellman, and Gamma, voiced by the Mac OS X voice synthesizer Fred), which are Literal Split Personalities of his original character, Church. In season 10, however, each AI has a different voice actor.
Lampshaded at a particularly memorable occurrence during a live table read where Matt Hullum must voice Sarge and the effeminate Doc conversing all while reading the script for the very first time. Bernie, who is voicing Vic for this, comments that the two of them sound alike (may have been an ab lib right there as that nothing like this happens in the actual episode), which 'Sarge' vigorously denies, and then tells 'Vic' that maybe he should call up Church or Lopez.
Niko Anesti voices most characters on GEOWeasel, including every character speaking prior to the eighth episode. Dr. Schnoz, the arch-rival, and Sapphire, who is female, are pitch-shifted down and up, respectively. When another female character who is actually voiced by a woman shows up, Sapphire's voice is explained as her having a male voicebox and using helium in an attempt to mask it.
In A Day With Bowser Jr's early days, almost all of the characters were voiced by Dannywaving, the creator. Over time, though, this issue lessened as the voice actors began to increase.
In Dr. Havoc's Diary, Carlo Moss voices Superpowered Man, Kyle the Henchman, and Frank the Shitty Boss.
In The Trash Pack webseries by Mukpuddy, all twelve characters are voiced by Ryan Cooper, one of the team members of Mukpuddy, each with a distinct voice.
DC Super Hero Girls definitely gets a lot of mileage out of voice acting. To go into more detail:
Tara Strong voices both Harley Quinn and Poison Ivy. Older fans might see this as a weird coincidence not only for the fact that they're both normally Batman villainesses, but...well. She also voices Raven.
Grey DeLisle voices Wonder Woman, Giganta, Silver St. Cloud, and Platinum. The Lego Brain Drain movie has her also voicing Lois Lane.