Deadly Rooms Of Death may sound like a FPS dungeon crawl, but it is actually a pure puzzle game — and an extremely good one at that. You can get version 1.6 (the "Architects Edition") for free from http://www.caravelgames.com/
. The current version, "Journey to Rooted Hold", costs US$20 and a year's subscription to CaravelNet costs US$12. The advantages of buying these are not obvious, even if you read the Caravel website repeatedly and play the demo of JtRH, but both are well worth it. (I expected the CaravelNet features to be valuable, but I was still suprised by how much CaravelNet adds to "the DROD experience.)
Great stuff. Great people. And a great community at http://www.drod.net/
... Highly recommended.