It's A Puzzle Wrapped In No, Wait, It's Just Puzzles
For those tabletop RPG gamers who like an intellectual challenge (or even folks who like brain-teasers away from the game table), Gizmodo has a nice collection of logic puzzles, most of which have decent hooks and neat solutions.
Be warned! Most of them are quite difficult. However, the underlying hooks on the bulk of them are generic enough at their core to adapt to the genre or setting of your choice. If they prove too challenging for the players, then a decent IQ roll should supply them with enough hints to still make the players feel smart when they figure them out fully.
-- Steven Marsh