I don't have the spare time these days to really sit down and finish games, but one series I've thoroughly loved is the Dead Space series. The third outing is currently being worked on and IGN is reporting that Dead Space 3 will feature drop-in, drop-out co-op!
In Dead Space 3 you play as Isaac Clarke, just like the previous two games, and if players choose, you can fight alongside a man with a scar on his face, an engineering RIG of his own, and glowing red eyes glaring at you from his helmet. Oh yeah. During the single-player campaign this character acts as Isaac's guide.
Do you think it sounds like a simple tack-on? Nope. The cooperative mode mirrors the single-player campaign, but Isaac's story changes to accommodate it. In one sequence within Dead Space 3, Isaac and this new character stumble, wounded, bloody, and missing chunks of armor, out of burnt wreckage together. If you were playing by yourself, this scene happens the same, but without the other character there.
Players can also work together with their skills such as telekinesis, to interact with pieces of the environment. You'll also be able to share ammo and heal your co-op partner, but there is no revive system. If one of you dies, both players have to reload from the last checkpoint.
Personally, I can't wait - this will be a day-one purchase/play for me. Plus, it seems I have a thing for "3" games... Max Payne 3, Diablo III... Half-Life 3? I wish.