The supporting actor and actress categories were introduced in 1936 to honor those actors whose characters don't drive a narrative but nevertheless play key roles within it. The additions also had the benefit of increasing the number of "glamorous" categories by two. For the first 5 years that the supporting actor award was in play, the category was pretty well dominated by Walter Brennan, who won three times for Come and Get It (1936), Kentucky (1938), and The Westerner (1940). He holds the record for most wins in this category.
This year's nominees:
Matt Damon, Invictus: This is Damon's 2nd acting nomination. He was also nominated as Supporting Actor for the Golden Globe, SAG, and Critics Choice Awards.
Woody Harrelson, The Messenger: This is Harrelson's 2nd Oscar nomination. He has also received nominations from the Golden Globes, SAG, and the Critics Choice.
Christopher Plummer, The Last Station: Shockingly, this is Plummer's 1st Oscar nomination. His work in The Last Station also received Golden Globe and SAG nominations.
Stanley Tucci, The Lovely Bones: This is the 1st Oscar nomination for Tucci, who also has nominations from BAFTA, the Golden Globes, Critics Choice, and SAG.
Christoph Waltz, Inglorious Basterds: This is the 1st Oscar nomination for Waltz, who has been winning every acting award in sight since Cannes. He seems unstoppable at this point, but there is of course always a chance that Oscar could go another way. In 2004 Thomas Haden Church was winning everything for Sideways but the Oscar ended up going to Morgan Freeman for Million Dollar Baby. If you attribute that win at least in part to sentimentality and a general feeling that it was "time" for Freeman to win, then maybe this year it might go to, say, Plummer? I doubt that will happen (or that it should), but it's not over until it's over.