(I didn't have much time to put this page together yet. I'll be adding more links in the future. If you have any suggestions, feel free to e-mail them to me).
The MIT Mystery Hunt - The largest, longest, and most difficult puzzle extravaganza in the US (perhaps the world). It takes place every January during Martin Luther King Day weekend and consists of over 100 puzzles and is solved by very large teams. You don't have to be an MIT student to compete. If you are interested in attending and are looking for a team to join, feel free to e-mail me for more information.
Brainbashers - A great site with a lot of puzzles of different types, ranging from word puzzles to Japanese-style logic puzzles.
The American Crossword Puzzle Tournament - Held in Brooklyn each year in March.
The National Puzzlers' League - The world's oldest puzzle organization.