Aegis Wing is free if you have an Xbox 360 and a connection to the internet.
Crafted by three interns at Microsoft, this is the sort of thing that redeems my Xbox 360 experience after I lose some gameplay due to a disc read error.
The game is fun, features cooperative multiplayer (both local and over Xbox Live), and is simple enough for pick up and play for anyone.
Did I mention that it’s fun?