Maurice LaMarche is the voice of The Brain. He is well known for his impressions of Orson Welles, and he worked alongside Rob Paulsen on Pinky and The Brain. Paulsen plays Pinky.
Here's an impressive list of credits for Maurice LaMarche.
"Sonic Underground" (1999)...........................Sleet
Wakko's Wish (1999)...................................The Brain, Squit
"Chimp Channel, The" (1999)..........................Harry Waller/Bernard the Sarcastic Cockatoo
Futurama (1999).............................................Kif Kroker
Dilbert (1999).................................................The Garbageman
Descent 3 (1999, videogame)..........................Various
The Howie Mandel Show (1998).....................Announcer
Pinky, Elmyra & The Brain (1998)...............The Brain
Home to Rent (1997).......................................N/A
Extreme Ghostbusters (1997)...........................Egon Spengler
Johnny Bravo (1997)........................................Various
Space Jam (1996)............................................Pepe le Pew
Captain Simian & The Space Monkeys (1996)......Dr. Splitz/Splitzy
All Dogs Go to Heaven 2 (1996)......................N/A
Dexter's Laboratory (1996)..............................Dick Mcman/Ratman
Hey Arnold (1996)..........................................'Big Bob' Pataki
Freakazoid! (1995)..........................................Longhorn
Carrotblanca (1995)........................................Yosemite Sam (as Gen. Pandemonium)
Pinky and The Brain (1995)..........................The Brain
The Tick (1995)...............................................The Deadly Bulb/Mr.Smartypants/Thrakrazogg
Twisted Adventures of Felix the Cat (1995)......N/A
Ed Wood (1994).............................................Orson Welles
The Critic (1994).............................................Jeremy Hawke
Animaniacs (1993).........................................The Brain, Squit
Cool World (1992)..........................................Mash
The Little Mermaid (TV Series, 1992)..............Scuttle
Taz-Mania.......................................................Hugh Devil
Attack of the Killer Tomatoes...........................Zoltan
Tiny Toon Adventures (1990)...........................Dizzy Devil
Dennis the Menace (1987)...............................George Wilson
The Real Ghostbusters (1986)..........................Egon Spengler
The Funny Farm (1983)....................................Dickie
Inspector Gadget (1983)..................................Chief Quimby
Rock & Rule / Ring of Power (1983)................Sailor