Maybe not as underated, but Cary Elwes has played a wide variety of different roles very well. He definetly has not gotten the credit he deserves.
I also second David Morse, William H. Macy, Jennifer Connely, and Don Cheadle.
I think the best, and most often times overlooked, are the actors who you are surprised to find out played both charactor 'w' in movie 'x', and charactor 'y' in movie 'z', even when the roles don't involve heavy makeup. I would put Sam Rockwell in that catagory.
I have to agree on Cary Elwes. I have always admired his work. I think his main limitation is a generally poor choice of movies. Not to gang up on the talent challenged, but I think Dracula would have been significantly better if he had played Keanu Reeves' character. Don't get me wrong. I don't think Keanu is entirely incompetent. He was good in Parenthood and River's Edge, but they are both the same character. C'mon Donny, you had to know you were setting yourself up.
Can't believe I forgot Macy! Another one that springs to mind is Sam Shepard - from 'Days Of Heaven' through 'The Right Stuff' and now 'Black Hawk Down', he always has great screen presence.