Play Binding of Isaac again. Seriously, unless you've gotten all the endings and items, you've probably still got heaps of things to do in that game.
What this dude said. The first few playthroughs annoyed the hell out of me, but then you start unlocking tons of new items, levels, and characters. Eventually it'll get to the point where you will still be up raging at 2am trying to beat Satan as the blue baby (or ??? or whatever it's called).
Personal recommendations for payed titles go to; Bastion, Sequence, Jamestown, Pixeljunk Eden, Serious Sam: Double D, Serious Sam: Random Encounter, Trine and Limbo. A.R.E.S isn't too bad either, though the soundtrack is easily the highlight. Oh yeah, Audiosurf is awesome. As a brawler fan I have to also list Shank, I love both the original and the sequel.
Although I have some of these on steam I haven't tried them yet; Dustforce, Fractal, Pineapple Smash Crew, Q.U.B.E, Dear Esther, SpaceChem, Trine 2, and Solar 2. They all seem great. Some have GB quicklooks, so maybe check those out.
@Landon: Loved the premise but I felt the implementation was really poor. The long-winded repetitive nature of the 'gameplay' really sapped my initial excitement, I did like a few spots in the story though. Like you said "its free", so its worth giving it a shot.
TO THE MOON!
I recently played through this little title, coming in with little to no expectations and being utterly and completely blown away. The game managed to tug on my heart-strings in every single direction plausible, making me laugh or somberly reaching for my guitar with the hint of dampness clouding my vision. Dramatic expressionism aside; it's a really, REALLY good game that deserves a look, at the least. There's a pretty sizeable demo that should give you an impression of what it is that you're getting yourself into. And if you want to continue, you can simply purchase the full game for the 10 bucks and continue. It's truly worth it in my opinion. And hey, an hour of it is already free, so why don't you give it a go?
That said, it's almost not a game, and more of a digitalized, interactive film. But it's one worth experiencing; just as long as you understand that you're not getting another Binding of Isaac here.
Otherwise, well, Bastion and Limbo, as well as Braid are all great titles.
anything from Dejobaan Games. Actually, let me narrow it down a tad. AaaaaAAaaaAAAaaAAAAaAAAAA!!! A Reckless Disregard For Gravity is pretty good, but they recently made an updated version for the same price that looks better and has more to do. If you're into Katamari and games like it (I assume there are other games like Katamari at least) you might like The Wonderful End Of the World, you basically play as a puppet for a goddess picking stuff up, it's interesting but short. If you're into weird music games, 1... 2... 3... Kick It (Drop That Beat Like An Ugly Baby) is okay, it's currently in a somewhat closed beta, in that you have to pay 10 bucks to get it, but if the company is having some smooth sailing right now a full version should drop within a few days. Also Bastion if for some reason you haven't played that yet.