A large percentage of indie games can fit under that criteria. XD
Hmm... This was technically gifted to me, sort of as a prize, though I did get to pick it out: VVVVVV
The idea of the game is that instead of jumping, you reverse your own gravity, so you're constantly flipping up and down. The gameplay is designed to focus on it specifically: there are no doors to be opened, no bosses to be fought, just the obstacles in the environment to overcome. And let me tell you, it does that really well; the obstacles are often challenging, but creative and still enjoyable. It's also got a great soundtrack, and users can make their own levels.
BIT.TRIP RUNNER is one that I actually bought myself. It's a type of rhythm game. You're constantly running forward, and there are obstacles and other elements of the game that match up with the timing of the game music. If you get past an obstacle or pick up a piece of gold, it makes a sound that adds to the music. If you pick up a core, it sort of levels you up, and the sounds level up somewhat, too, but never so much that they become overwhelming. Also challenging, also fun.
RUNNER 2 is one I'm planning on getting soon. It's basically like an enhanced version of/sequel to BIT.TRIP RUNNER (though it's technically not part of the BIT.TRIP series). I followed the development blog and it looks fun!
Anyway, BTR and VVVVVV are both really fun and I would recommend them both. They both have demos you can try if you want to just give them a look. (Not sure if BTR has an official one outside the Wii Shop Channel, but feel free to look.)