Damnit, Dave, I opened this thread specifically to mention Bernadette and you beat me to it 
Jim Bakkus was the voice of Mr. Magoo… and don’t forget Boris Karloff narrated How the Grinch Stole Christmas.
(I think I just dated myself here)
Patrick Stewart did the voice of one of the military guys in ‘The Plague Dogs’.
I remember hearing Tone-Loc’s voice in a cartoon once. Can’t remember what show, though.
You’re probably thinking of “Ferngully”, frock, though he also did a voice on “Titan A.E.”.
His voice is solid and impressive, but I don’t think it’s very flexible. Whatever he does, he sounds like Tone Loc–at least, so far as I’ve heard.
She uses the same voice for the kid in Time Squad.
Buzz Lightyear of Star Command has also used a number of celebrity voices:
Buzz: Patrick Warburton (Seinfeld, The Tick)
Nova: Nicole Sullivan (MAD TV)
Booster: Stephen Furst (Animal House, Babylon 5, St. Elsewhere)
XR: Larry Miller
Emperor Zurg: Wayne Knight (Seinfeld, Third Rock From the Sun, Jurassic Park)
Warp Darkmatter: Diedrich Bader (The Drew Carey Show
French Stewart: Rentwhistle Swack (“Harry” on third rock)
XL: Bobcat Goldthwait
Bill Mumy: Eon (Lost in Space, Babylon 5)
Jonathan Harris: Era (“Dr. Smith” on Lost in Space)
(Mumy and Harris appeared in the same episodes together as aliens subjecting Buzz to experimentation. This is the first time they worked together since Lost.)
Marl: John Kassir (“The Cryptkeeper”)
This is not the complete list.
If you want the complete list for the guest voices on Buzz, click here.
Another person who was a celebrity before he did cartoon voices is Paul Winchell, known as the voice of Tigger, Gargamel and Dick Dastardly. Previously, he was known as a ventriloquist and had his own TV series in the 50s.
(That is a link to his website. Read it. Did you know he was also a doctor who invented an artificial heart and the disposable razor? Unfortunately, he listened to the people who assured him that no one would ever buy a disposable razor.)
Here’s an interesting take from our malcontents (I mean that as a compliment) at Teevee.
Most of the obvious ones have already been mentioned, but I could fill in some the blanks.
You know that starfish on Spongebob Squarepants, Patrick? That’s Bill Fagerbakke, that lovable lug Dauber from Coach. He also does a voice on Lloyd in Space. He also did voices on Gargoyles (Broadway), Jamanji (as the Robin Williams character), Hercules (Cyclopes), and I’m sure he did some “cameo” and one time roles on other cartoons.
If you hear a boring professor-type guy in a cartoon, it’s probably Ben Stein. Before you had a chance to win his money, he was a speech writer for Nixon and Ford. Like most former White House officials, after his stint in politics we went into opinion columns. He still pens an occasional one for the Washington Post’s Outlook sction. But, his Hollywood career pretty much began when he bored the nation as Ferris Bueller’s Economics teacher. Thus having found his niche as the monotone unemotional scholarly beurecratic dorky character, he started showing up everywhere in Hollywood in both live-action and animation.
His longest lasting role was probably as Dr. Ben Stein, one of Duckman’s neighbors on Duckman. I remember him most as Francis Pumphandle (“But everyone calls me ‘Pip’.”) on Animaniacs. He also did voice work on Bruno the Kid (already mentioned), Hercules, The Mask, and Earthworm Jim.
Ed Asner, in addition to the work already mentioned on Gargoyles, Freakazoid, and Spiderman, was one of the original voices on Captain Planet, was Daggett on Batman: TAS, and he probably guest voiced on dozens more.
Keith David has a very distinctive voice, which I will always recognise as Spawn (Spawn) or Goliath (Gargoyles). Now he’s on The Job with Denis Leary, but I’ve heard him it the Final Fantasy movie, and the US dub of Princess Mononoke.
You can’t make a cartoon without Jeff Glen Bennett. I’m not even going to try to list his resume.
Going back to kiddie shows, Bill Cosby produces one called Little Bill. Since it is a Cosby production, by law a part must be cast for Phylicia Rashad. Gregory Hines and Ruby Dee’s in there, too.
Anyway in regards to the voice-over business,I can classify the players into three groups. There are the ‘professional’ voice actors for whom the vast majority of their work is simply cartoon voices. Jim Cummings, Jeff Bennett, Tres MacNellie etc Unless they get work on the big series like The Simpsons, their pay is about so-so. Thus why they take many jobs. Then there are the big celebs who take roles in cartoons and get paid handsomely. More so if it’s a big blockbuster Disney movie. Then there’s the mid-tier actors who fit inbetween the two ends of the business. They tend to have supporting roles in TV shows or movies. They tend to get paid pretty well, though they don’t headline the Disney summer posters.
This is obscure? I went into “Gaaaaaaaaaaah, Donny Osmond!” fight-or-flight mode and almost clawed my way out of my seat as soon as he started singing!
My favorite obscure Disney voice reference is Kenneth Mars. His most famous live action role is probably that of Franz Liebkind (the Nazi scriptwriter) in The Producers, but he has a very impressive list of voice work credits to his name. Among many others, he was the voice of King Triton in The Little Mermaid. He continued the same role in the direct-to-video sequels and television spin-off of that movie.
Dan Castellaneta was also Earthworm Jim in the cartoon of the same name.
Someone mentioned Casey Casem as Shaggy on the Scooby-Doo cartoons…he also voiced Robin for the Superfriends. I remember being about 9 when I clued in that Shaggy and Robin sounded the same.
Watching them now (lo, these 22 years later), every time I see Shaggy talk I try to picture Robin saying those lines, and vice-versa. It gives Scooby-Doo a whole new dimension.
Jerry Doyle (Garibaldi from Babylon 5) was Captain Simian in Captian Simian and the Space Monkeys.
Hey, it’s better than it sounds.
I think Michael Dorn is the bad guy, too.
I can’t believe no one else has mentioned Fat Albert. Besides hosting the series (one of the longest-running Saturday-morning cartoon series), Cosby was the voice for five different characters: Fat Albert, Bill, Mushmouth, Mudfoot and the Brown Hornet.
Oh yeah, and the voice of Patrick on Spongebob Squarepants is the same guy who played Tom Cullen on “The Stand.” M-o-o-n, that spells Patrick. 
Yeah, and Dom Irera was Spider, James Avery was Gorilla… I can’t remember who played Shao Lin, or if she was famous outside voicework or not…