Amazon is having a sale this week on Xbox 360 titles. If you purchase one game at normal price you'll get a second game at 50% off. There's a lot of solid co-op titles on the list like Portal 2, Left 4 Dead 2, Saints Row 2 and Earth Defense Force: Insect Armageddon. Surprisingly the recently released Catherine is also on the list as well.
The full list of titles that qualify for the promotion can be found here.