This is a thread to discuss some of your other favorite puzzles, riddles, brain teasers, armchair treasure hunts, etc.
To get started, here's an online puzzle hunt I've participated in before: Puzzlecrack. It's a week-long competition with clues given through the web page. Past competitions (and solutions) are still there for you to figure out.
Another similar one is Microsoft's College Puzzle Challenge.
Any other favorites?