VVVVVV is a new indie game developed by a guy called Terry Cavanagh. It's a rather simple platformer with nice gameplay and some fresh ideas. In short it's awesome.
It's not free (costs like 10 bucks), but you can try out the demo for free at:
http://thelettervsixtim.es/You can also buy it from the same site.
Here's a gameplay video:
While the game is pretty short, there's lots of extras that can take you a lot of time (and patience) to do so it's definitely worth its price.
Give it a try!