20XX is a game built to feel like Mega Man X, but with random levels, random powerups, permanent death, and multiplayer. The game uses a procedural level generation algorithm that pieces together levels from a certain number of mechanics at a time; it switches rapidly between playing like an action game and playing like a platformer, mixing precision jumping with fevered combat.
Add in permadeath and over 100 powerups to find in between agonizing deaths and you've got hours of nail-biting jumping-and-shooting goodness on your hands.
Also, 20XX features full co-op. Play with your buddy on the couch or over the interwebs.