GUM - Super simple mechanics but incredibly well presented. An updated version was released yesterday, in fact!
Jelly No Puzzle - Super simple but incredibly difficult.
Alan Hazelden's Puzzlescript collection - A varied collection of well designed puzzle games.
Corrypt - Here's where puzzling gets really meta. Thread with care.
English Country Tune - Extremely varied and difficult puzzle game. Also check out Sausage Roll.
Art of Balance - Enjoy the fishy physics (fishics?) and stack blocks preventing the tower from falling over. Has less strong puzzle design, but is backed up by good multiplayer.
Blockwick 2 - Simple, fun block shoving puzzle game. Some gimmicks are weird, but overall really smart design.
Sokobond - +1 for the scientific accurateness, has some awesome puzzles. Also check out A Good Snowman.
If you know any others, let me know! In the meanwhile, I'll finish my last tweaks for Tahira and blow you all awake with it tomorrow!