True Romance was a great movie. Also, Four Rooms with Antonio Banders and directed by Quentin Tarantino was awesome.
The one movie that I always bring up when a topic like this comes along is Stranger Than Fiction with Will Farrell. It's really a great little film that most people have never seen or hated because they were expecting a comedy. It also shows that he can portray someone other than a complete retard. I also think Funny People was vastly underrated because people went in with expectations of a raunchy comedy true to Apatow's form. Also, when making these types of lists, let's not confuse underrated movies with movies that no one has ever seen. Underrated movies are movies that most have seen and hated.
Requiem For A Dream, Memento, The Saint, and Wet Hot American Summer come to mind. I don't think any Tom Hanks or Tarentino flick can be in a category of underrated.
Roadhouse. I'm not a big movie buff...But, I noticed this movie was on TV, today. I've seen it a few times...Great movie.