Bill Murray in Zombieland is one of my favorites. I don't think Tom Cruise as Les Grossman in Tropic Thunder qualifies but if it does, that is a good one too.
Anchorman had a lot of good cameos too with the rival news gangs. Ben Stiller, Vince Vaughn, Tim Robbins, and Luke Wilson were the heads of the gangs.
Alfred Molina in "Boogie Nights". <iframe title="YouTube video player" src="http://www.youtube.com/embed/H9pZpcxodf4" allowfullscreen="" width="640" frameborder="0" height="390"></iframe>
Tough call between the two. That scene between Walken and Hopper is one of the best ever. His cameo in Pulp Fiction was priceless, though obviously didn't call for anything game-changing.
Quentin Tarantino in........................nah, I'm just kiddin' :: Robert Duvall in "Apocalypse Now".