Definitely my favourite "pure" puzzle game, DROD features countless brilliantly designed, extremely challenging puzzles. As a bonus, there's the visceral pleasure of slashing through whole armies of evil creatures singlehandedly. If you're a hardcore puzzle fan and you're getting bored of easy stuff like quantum physics, you must try this game!
Note: the download was removed at the request of game author Erik Hermansen. If you want to try it, download the much prettier - and free - remake called DROD: Architects' Edition on this site coded by Mr. Hermansen himself :)