I’ve never heard anything bad about Henry Winkler or (the late) John Ritter.
Nor Tim Robbins – unless being highly and overtly political automatically disqualifies him. Same with Michael Moore (certainly a “celebrity”) – the content of his films is controversial, to say the least, but I’ve never heard any negative gossip about his personal life.
Jackie Chan seems really nice, even though this weird thing with him going on stage with some guy just happened. He seems like a quality guy overall.
Never heard anything bad about Emma Thompson, or Anthony Hopkins (who is supposed to be extremely professional and actually learns his lines) or Robert Duvall.
One of the first names on this list should have been Ron Howard.
I have only heard good things about William H. Macy.
Except for the whole adultery thing.
I had the pleasure of meeting Elizabeth Montgomery when I was 15. She was a totally classy, classy lady.
Jim Henson.
Now there was a nice man. He left us way too soon.
There was a cool story related in the book Opera For Dummies. I don’t remember the specifics, but here’s the gist of it.
The great tenor Enrico Caruso was performing onstage with another male singer (a bass, I believe). At some point the bass realized that his own voice was just about to give out. He looked over at Caruso, agony clearly visible in his eyes, and Caruso realized exactly what was happening. Without missing a beat (literally) he crossed over to the other singer, put his arms around him in a manly bear hug (his own back to the audience) and sang the remainer of the bass’s aria for him while the bass mouthed the words. Not even the conductor noticed what was going on.
Now that’s a class act.
I met Julia Stiles once in a restaurant in Chicago. She was really nice, very approachable - in fact she started talking to me, we spent 15 minutes joking about waiting in lines, then we traded dirty limericks.
Here’s a link. Guy just embarassed himself a little bit.
I’ve never heard anything bad about Clint Eastwood. Also, he brought Flat Stanley to the Oscars.
I’ve heard that Steve Buscemi is a nice guy.
The whole Sondra Locke episode was very, very ugly, though who’s more at fault is hard to say.
A group of Oprah audience members got swept off to Hollywood for some sort of event a few years ago and reported that Wesley Snipes went out of his way to hang out with them and spent a lot more time with them than any of the other stars did.
Forgot about that. Well, there’s still Jackie Chan, sober. 
But who took over the tenor part for Caruso?
My only basis for these choices is what I see on TV, but:
Ellen Degeneres
Dolly Parton
LLCoolJ
All seem like really nice people.
As for people I have met and they really are nice:
Amy Grant
Well, except for that disturbing fetish for jamming his fist into animals.
Dolly Parton
Ewan McGregor
Audrey Hepburn
Gregory Peck
Cary Grant
Mark Hamill
Clark Gable
Matthew Perry
Viggo Mortenson
Liv Tyler
Renee Zellweger
I’ve heard that Patrick Duffy is a nice, unassuming guy who gets along well with fans and co-workers and gets quite a few parts because he’s so easy to get along with.
Another thing I admire in Jodie Foster is the ability to keep her private life private. I usually don’t read the celebrity magazines, so I may be totally out of touch, but I get the impression that she manages to raise her child(ren?) out of the public eye, and show that a major Hollywood star doesn’t need to put her (or his) personal life on display.