Off the top of my head, I can think of Leonardo DiCaprio, Will Smith and Denzel Washington.

Definitely Tom Cruise but the one supporting role he did do (Magnolia) was one of his best performances ever.

Anybody else?

+ Tropic Thunder

Brad Pitt
George Clooney

Harrison Ford

Clooney won a best supporting actor oscar for Syriana
He also played a supporting role in 'Good Night and Good Luck'.

Burn After Reading
Snatch
Ocean's 11, 12, 13
Fight Club
True Romance

The Departed
What's Eating Gilbert Grape

Bad call on Clooney and Pitt. Nevermind.

Tom Hanks

I'd say Leo was the lead, or one of the leads at least, in The Departed.

Absolutely.

Cary Grant, James Cagney, Henry Fonda, Spencer Tracy, Montgomery Clift, John Wayne, Clark Gable, Gary Cooper.

Greta Garbo, Hedy Lamarr, Joan Crawford.

Jet Li, Chuck Norris, Jean-Claude Van Damme.

Toshiro Mifune.

Jean Gabin.

Bugs Bunny.

Yeah, definitely not Pitt and Clooney. I would say they are such amazing actors because they take on these great supporting roles.

Harrison Ford wasn't really established whenever Apocalypse Now was filmed.

Was Tropic Thunder a long cameo or a supporting role?

I wouldn't say Pitt was a Supporting character in Fight Club and DiCaprio was definitely not supporting in The Departed. And DiCaprio wasn't established whenever Gilbert Grape came out.

Supporting role, IMO. The part felt too large and effected the story too much to simply be a cameo.

Plus he's credited which takes him out of cameo status (IMO).

Edit:
Of course, that's not always true, since he was credited in Goldmember as well, so, yeah MrSmearkase's definition seems to be better, lol.

With the exception of Where Eagles Dare (in which i think he was certainly one of the stars) then i would say Clint Eastwood. How could you give the guy a small role though.

Clint began his career in bit parts at Universal in the '50s. He's most famously spotted as a bumbling lab assistant working for John Agar in REVENGE OF THE CREATURE (sequel to CREATURE FROM THE BLACK LAGOON). He's one of the pilots who drops the bombs on the title monster in TARANTULA. He's one of the group of sailors supporting Donald O'Connor and Francis the Talking Mule in FRANCIS IN THE NAVY. He had a bigger part in a western called AMBUSH AT CIMARRON PASS (1958). He started "Rawhide" the following year.

(Trivia note: in AMBUSH AT CIMARRON PASS, Eastwood plays a character named Keith Williams, which happens to be the real name of current actor Keith David. Keith David acted for Eastwood in BIRD.)

Not much of a star now days, but Jim Carrey, only one I can think of is Batman Forever.

Earth Girls are Easy, The Dead Pool, Pink Cadillac, Peggy Sue Got Married...

Don't forget Simon Birch, which came out long after Carrey had become a household name.

Kate Winslet doesn't do many supporting roles, especially since Titanic.

Sorry to get off the beaten path a bit but I have a Tom Cruise question.
I read a comment on Youtube that he was some character in Young Guns that got shot in the back. Is that true or just bullshit?

It's true. They slapped a big mustache on him, but if you're looking for him it's pretty obvious.

Video here: http://www.youtube.com/watch?v=99n5o6P1_Yo

There's also American Graffiti and The Conversation but I wouldn't count those since he wasn't "established" at that point.

I can't recognize anyone from that video, but I'm sure it's more obvious on the DVD/Blu-Ray. Thanks for the confirmation though.