In honor of the Game Developers Conference this week, QCF Design has opened access to the beta version of Desktop Dungeons. Toss them an email address to sign up and you'll be able to play for free until March 31.
While I wouldn't call Desktop Dungeons a puzzle game, it can certainly feel like you're in the midst of solving puzzles while playing. I would, however, call it one of the best gateway titles to try out should you be interested in rougelikes. The original freeware release is still good, but there's enough going on with this commercial release that they're both very much worth playing.