ItsJustSomeRandomGuy does all of the male voices in I'm a Marvel... and I'm a DC. Mostly, he's very good at making each voice different — with the exception of the strangely gentle Captain America voice, they're all similar, but distinct. However, when characters are worked up they all sound the same.
The internet radio drama Fobbies Are Borange, at one point, had a voice actor have a knife fight with himself.
Every character in Reynaldo The Assassin is voiced by the same man. This is very noticeable at some points.
LittleKuriboh of Yu-Gi-Oh! The Abridged Series. Also by extension any Abridged Series. LittleKuriboh gets extra points though for doing a live-action reenactment of his first episode. Basically him being taped on a street in England switching voices/characters from one second to the next.