As others have brought up in this thread already Diggstown, Memento, Lock Stock and Two Smoking Barrels and Snatch. I find it weird that Memento would be underrated. How long will it take for people to understand Chris Nolan is amazing?
I would add Boondock Saints and High Fidelity to the list. Two of my absolute favorites. Swordfish is underrated for probably the best opening 15 mins, it grabs you by the balls and you are hooked. As far as a recent underrated movie, Dredd was probably the most underrated movie I saw in the last year.