It looks like I spoke too soon, another great co-op title just popped up on Amazon.com for a discounted price. Doom, sporting four player split screen and online co-op, is available for less than $3 on Amazon's website.
The classic game defined the co-op shooter, and is available on more than 25 different platforms. The Xbox Live Arcade version is the only version that natively supports internet play with voice chat. Trust me, you'll want to hear your buddy scream when you "accidentally" shoot him in the face with a rocket.