That move Just Friends was focused around a Ryan Reynolds/Amy Smart romantic comedy but Anna Feris stole the show as the psychotic actress.
The unfunny and misguided In & Out starring Kevin Kline was just stupid. However, the bride left at the altar played by Joan Cusak was hilarious and made the movie worth watching.
One I haven’t seen mentioned yet: Tommy Lee Jones in The Fugitive. The title tells you that you’re supposed to be focused on Harrison Ford as the escaped convict trying to prove his innocence, but Tommy Lee owned every scene he was in.
“I didn’t kill my wife!”
“I don’t care!”
Altho also not a small role, Renee Zellweger in Cold Mountain as Ruby. I definitely prefer her in this type of supporting or at least 2nd-billed role–she also was the best part of “White Oleander” in my book, playing the uber-tragic Claire without making her a caricature, and she was bettern’ Cruise in Jerry MacGuire, as well. On the other hand, she was outshown by both Catherine Zeta-Jones and Richard Gere in “Chicago”, IMH
John C Reilly is one character actor who I feel nearly always steals the show—he positively made this year’s Will Ferrell vehicle “Talladega Nights” worth watching (& in fact a much more amusing film than I thought it would be) for me, and was also lovely in “Chicago”, “A Prairie Home Companion”, “The Good Girl”, & “The Hours”.
And h.sapiens, Geoffrey Rush is the only reason I am going to see “Pirates 3”, frankly. I LOVED him in the original & also thought he darn near stole Depp’s magic.
–Beck
I’m surprised that this far into the thread that no one’s mentioned Kevin Spacey in Se7en.
Though he didn’t totally steal the show, Benicio Del Toro made the most disposable character in The Usual Suspects into one of the most memorable (if not always decipherable).
He’ll flip ya. He’ll flip ya for real. 
Ooh! Geoffrey Rush is to be in POTC3? I am so there. I love Geoffrey Rush!
Here are some more:
I’m not going to list all the movies he has been in, but Jim Broadbent totally outshines everyone in every movie he’s in, IMO.
Alien is a favorite film of mine, and I like everyone’s performance, but Ian Holm shows the rest of the ensemble what acting is all about.
I love the hokey movie The Birdcage, and think everyone did a bangup job in it, but Hank Azaria as Agador Spartacus equalled or even exceeded Nathan Lane’s performance, I think. “You are afraid of my Guatamalan-ness!”
Has anyone mentioned Alan Rickman in Die Hard yet?
Gollum.
More Sam Elliott:
Roadhouse (someone had to steal it)
Mask
Vanessa Redgrave in Mission Impossible
Linda Hunt in
The Year of Living Dangerously (playing a male–take that, Crying Game)
Let me put forth Peter Sellers in films like:
Lolita - He was at best the fourth lead in that but he was hypnotic.
The World of Henry Orient - While he had the title role, he was at best the third lead. But he was dominating.
and finally
The Pink Panther (the original)- This was to be a David Niven vehicle, with teen heart throb Robert Wagner and sexy Claudia Cardinalle (sp) picking up the slack, but all anyone remembers is Peter Sellers.
For anyone who’s interested, this awesome performance is on YouTube.
Benicio Del Toro also stole Fear and Loathing in Las Vegas from Johnny Depp
What about Oliver Platt. This is a fun throwaway movie. Bridgette Fonda was very good too. But she was the star.
There’s something about minor actors with a lot of character:
**Sean Hayes & Megan Mullally ** stole Will & Grace right from under the stars’ feet.
**James Marsters and Juliet Landau ** (Dru & Spike) stole every scene they were in on BtVS.
Michael McKean and David Lander (Lenny and Squiggy) (“Hello.”) stole the show from Laverne & Shirley.
Elle Fanning (Dakota’s little sister) in The Door in the Floor. For one so young (she turned five during filming), she is fantastic without being cloying or precocious.
I have to see this movie, because I can’t picture Hume Cronyn as anything other than a mousy nebbish bookkeeper-type. I’m experiencing severe cognitive dissonance.
Philip Seymour Hoffman in “Along Came Polly”
Neil Patrick Harris in both “Harold and Kumar Go To White Castle” and on the TV show, How I Met Your Mother.
What, you never saw the two episodes of Hawaii Five-Oh where he played “The Monopoly Thief”?
classic stuff!
Claude Rains in Casablanca
Rod Steiger in Oklahoma
Stubby Kaye in Guys and Dolls and L’il Abner
Ann Miller AND Betty Garrett in On the Town
Rita Moreno in The King and I
True, a lot of those parts were written to be scene stealers. But, particularly with Steiger and Moreno, the actors did a lot more with their performances than was in the script.