Someone mentioned Oliver Platt…when I used to watch Huff on Showtime, he stole every scene he was in.
My daughter mentioned Wallace Shawn in The Princess Bride.
Wilford Brimley in The Verdict.
The thought of Lander’s “Hello” still cracks me up.
Robert Knepper as T-Bag in Prison Break. He owns that show.
I just watched Nacho Libre last night and even though this movie is so not funny, it is totally worth watching just to hear Esqueleto say “I look hideous” after their first wrestling match. I am still laughing about that line!
Matthew McConaughey in Larger Than Life as Tip Tucker. The only redeeming aspect of a terrible movie.
I recently saw the original To Be or Not To Be, which starred Jack Benny and Carole Lombard. The show was stolen lock stock and barrel by character actor Sig Ruman as Colonel Ehrhart. He didn’t really have a lot of gag lines; he did it all with delivery, facial expressions, etc. Completely walked away with the movie.
42nd Street starred the pretty Ruby Keeler, but the glamorous Ginger Rogers beat the tar out of her. She could outdance, outsing, out-act, and out-wisecrack her with one hand tied behind her back.
Walter Brennan, always a character actor and never a leading man, easily stole scenes from Humphrey Bogart, Bob Hope, Spencer Tracy, and other big-draw names. For that matter, so did William Bendix and Jack Carson.
Oliver Hardy stole the show from John Wayne in The Fighting Kentuckian, probably one of the only western movies where I ever found myself rooting for the French.
Actually, now that I think about it, I can’t think of any westerns where I was rooting AGAINST the French either…
In El Dorado, John Wayne kept having the scenes stolen by Robert Mitchum and James Caan. Also, I kept getting distracted by Michele Carey, playing Josephine “Joey” MacDonald, the firey tomboy, though I’m not sure if she was stealing scenes for anyone else.
Another oldie was Flying Down to Rio from 1933. It starred Dolores Del Rio and Gene Raymond. Remember them? No? How about the two people in secondary roles: Fred Astaire and Ginger Rogers? They absolutely chewed up the scenery and the movie launched a huge movie career for the pair.
By the way, a very minor bit player in their second film was Betty Grable, but they didn’t allow her any screen time to speak of.
I guess you could consider that a “Western,” though it was really more of a “Southern,” being set along the Gulf coast of Alabama.
Of course, Alabama was more or less the western frontier at the time. 
From reading the reviews at rottentomatoes, it looks like Jennifer Hudson (an American Idol also ran) completely walks away with Dreamgirls.
Lee Marvin in a cast all trying to upstage everyone else in the Magnificent Seven proved himself the master scene stealer.
Yul Brynner is holding a conversation centerstage but all you notice is Marvins restlessness behind him.
And in Pat Bennetars music video “Life is a Battlefield” the slimy over thirty character who tries to pick her up in a city club but is rejected in favour of “the Kids” is so overwhelmingly the coolest dancer, that without even trying to it undermines the basis of the whole story.
Thomas Sangster in Love Actually.
The wife and I saw The Holiday last weekend, and Eli Wallich taught the newbies (Jude Law, Jack Black, Kate Winslett, Cameron Diaz) a thing or two about scene-stealing.
I agree with a lot of the above:
Alan Rickman (Hans Gruber) in Die Hard
Val Kilmer (Doc Holliday) in Tombstone
Johnny Depp (Agent Sands) in Once Upon a Time In Mexico
Jack Black (Barry) in High Fidelity
I should also add:
Bill Murray (Peter Venkman) in Ghostbusters (he had top billing, but it was still an ensemble cast)
Christopher Walken, Dennis Hopper, and Gary Oldman in True Romance (in their respective scenes)
Samuel L. Jackson (Jules Winfield) in Pulp Fiction
Will Arnett (GOB Bluth) in Arrested Development
Brad Pitt (Mickey) in Snatch
Vince Vaughn (Lester Long) in Clay Pigeons
Cameron Diaz in The Mask (her first role, and she’s never been more stunning)
Bill Nighy is the only reason I don’t regret buying a ticket to The Hitchhiker’s Guide to the Galaxy (2005).
Actually, I think the casting in that movie was across-the-board excellent. I just don’t think the movie was the least bit memorable.
Quite an accomplishment, considering that Lee Marvin **wasn’t in ** The Magnificent Seven!
In Friends with Money, Frances McDormand steals the show as the chronically angry person. (Maybe that’s not so hard to do, but this woman tends to steal scenes in many films.)