Casey Affleck, The Assassination of Jesse James by the Coward Robert Ford
Javier Bardem, No Country for Old Men
Philip Seymour Hoffman, Charlie Wilson's War
Hal Holbrook, Into the Wild
(not a great clip of Hal, but it was all I could find)
Tom Wilkinson, Michael Clayton
(again, this is just the trailer, but there are some good clips of Wilkinson in it)
What "counts" as a supporting vs. a lead performance could be debated endlessly. The bottom line is that the studios generally choose by pushing a performance for whatever category they think is appropriate. In this race then you have Casey Affleck, who was a revelation in The Assassination of Jesse James by the Coward Robert Ford, but that was really the lead role of the film, making it hard to compare it against the others, all of whom were good. Hal Holbrook's character was the most memorable of those Chris McCandless encountered on his journey in Into the Wild. Philip Seymour Hoffman was the best part of the otherwise so-so Charlie Wilson's War. Tom Wilkinson's mania actually added a bit of levity to the otherwise serious Michael Clayton. Javier Bardem was excellent in No Country for Old Men as a psychotic hitman. Genuinely creepy. I think he's going to win, but I'd probably still vote for Casey.