When I think of underrated actors, Leonardo DiCaprio is one that comes to mind. He's been in so many great movies, movies in fact that if you take him out of the role and place in a different actor, I'm not sure it would have gotten as much acclaim. He made the Aviator the great movie that it was (not to take anything away from Cate Blanchett). Tom Hanks and DiCaprio in Catch Me if you Can was a case of the actors making the movie, again. He was terrific in The Departed, he was great in Inception and Shutter Island. Even if the movie wasn't superior, his role in J. Edgar was again played well. He was sadistically entertaining in Django Unchained. Yet ever time, people pass him over, having not won a single award. Not one. An underestimated actor, an underrated actor, if there ever was one.
A few others that come to mind are Jeff Daniels, James Cromwell, and I agree with the above of Lisa Kudrow.