Steam has Halo: Spartan Assault (Digital PC) for $1.04 downloaded. They also have Halo: Spartan Bundle for $1.74.
Ends at 7PM PT. Steam has Red Wings: Aces of the Sky (Windows PC) on sale for $0 downloaded. This comic book action game set against a WWI backdrop features the legendary Red Baron, aerial dogfights and co-op mode.