Someone pointed me to this fascinating Web game, apparently "the hardest riddle on the internet". The goal of each puzzle in this game is to get to the next puzzle, and unlike most computer games, you can't win simply by applying the principle of banging your head against the door until it gives way. I'm now at level 8, and there's 134 levels (and growing).
Notpron is a refreshing change from games like Crimson Room, which mostly gauges your ability to point and click. If you play this last game, be warned: there's a URL in it that refers to a nonexistent page.