IAmSinistar: PixelJunk Eden which isn't on GOG (yet?).
Grargar: For some reason that I'm not too sure, the game is offered only as a Steam key in all the shops that is available, unlike PixelJunk Shooter and Monsters, which are DRM-Free on the Humble Store, aside GOG.
Speaking of the PixelJunk series, there is also a (hidden) discount on GOG for both of them.
PixelJunk Monsters HD -90%/$0.79
PixelJunk Shooter -85%/$1.34.
Stealth sale! These are awesome games, and PixelJunk Monsters in particular hasn't gotten a fair shake around GOG methinks. Go buy PixelJunk Monsters, people. At $.079 it is basically free, and it's an incredible game, one of my all-time favorites in fact.
I'm also waiting for Eden to become DRM-Free. That's my second-favorite PixelJunk / Q-games game behind Monsters.