Indiegala are giving away free Steam keys for Pixel Puzzles: UndeadZ.
"a traditional style jigsaw puzzle game with top down shooter gameplay thrown in to ruin your day and peaceful puzzling experience."
Apparently the Zombie shooting is optional and can be turned off if you just want to concentrate on the puzzles without worrying about Zombie attacks :)
User reviews are 'Mostly Positive' (recent reviews 'Very Positive') and the game has Steam Trading Cards.
You'll need a (free) Indiegala account, and if you haven't already done so you may need to link it to your Steam account and/or join the Indiegala group on Steam.
I can't wait until the day someone invents a robot that knows how to use a mouse…