Twin stick shooter and Steam favorite Beat Hazard got a downloadable content pack last month that adds two player online and local co-op. For a mere $4.99 (US) you can upgrade to Beat Hazard Ultra.
For anyone who doesn't already have Beat Hazard, you can purchase the Complete version for $12.99 (US). This includes the full game, the Ultra content, and support for iTunes.
If you would like more information about the game, be sure to read our pre-co-op review.