If you don't know his name
you've heard his voice
Most popular as Brain on Pinky and the Brain with a voice that sounds eerily like Orson Welles (he did Orson's voice in the movie Ed Wood in fact.)
Has really dozens of voice-over credits in various animated series--He's Kif Kroker and (I think, although I can't prove it yet) Calculon in Futurama and played the garbageman on the ill-fated Dilbert cartoon. His IMDB writeup goes into horrific detail about his career. He's apparently very adept at imitation and mimicry.