Developer: Creative Patterns
Platforms: Windows, Xbox 360
I can't believe I haven't suggested this already.
Quadsmash is a physics platformer originally released on the Xbox 360 Indie Games Channel. I picked it up a year or two ago for 400 points ($5), and it's easily my most played Xbox indie game.
The game has great, high quality art and music, and the platforming gameplay in the single-player campaign is surprisingly deep. There are a couple of neat mechanics (like the grapple beam), but what makes them stand out is that they can be used in multiple ways, for different things.
The single-player game mode consists of 10 progressively challenging stages which should keep you occupied for a couple of hours; despite the length, in my opinion the game is totally worth the $5 you'll pay for it on XBOX just for the solo mode. However, that's not where this game really shines.
This is hands-down the best multiplayer you can get for under $10. I've only had the opportunity to play with one other person at a time, but we've sunk hours and hours into the three multiplayer game modes. I can't really describe how awesome it is in text; you'll just have to check out the demo and see for yourself. The multiplayer more than justifies the $8.22 price tag for the PC release, provided you have at least one other person to play it with.