Alot of these guys have been mentioned but there are several missing from previous posts
In no particular order - these guys are REAL actors that absorb and become whatever role they play. Most of these guys are pretty famous but they are "Character Actors" by every sense of the words: (google is my friend
)
Denzel Washington
Gary Oldman
Tim Roth
Robin Williams
Steve Buscemi
Edward Norton
Samuel L. Jackson
Dustin Hoffman
Harvey Keitel
John C. Reilly
Sean Connery
Deniro
Pacino
Tom Hanks
Tommy Lee Jones
Gene Hackman
Jack Nicholson
Morgan Freeman
Ed Harris
Christian Slater
George Clooney
Laurence Fishburne
John Travolta
Nicolas Cage
Anthony Hopkins
Matt Damon
Tommy Lee Jones
Delroy Lindo
Richard Gere
John Malkovich
Kevin Spacey
Christopher Walken
Up and comers breaking out (past few years):
Terrence Howard
Clive Owens
Joaquin Phoenix
Jamie Fox
Andre Braugher
Heath Ledger
Philip Seymour Hoffman
Jeffrey Wright
EDIT: Robert Downey Jr. belongs on the list - no doubt ... dude's got "chops"!