Bangai-O Spirits is packed with amazing features -- the level editor, the Sound Load system, the shooting of stuff. But as if that weren't enough, it also has co-op. It's local, and limited to two players, both of whom require their own cartridge, but everyone should probably have their own Bangai-O Spirits anyway.
You can be joined by another shooty robot in any stage in Free Play -- which means any stage. This includes stages you've created yourself or transferred via Sound Load. IGN's Daemon Hatfield reports a lot of slowdown, especially in multiplayer, but slowdown in Bangai-O games is inevitable given the nature of the super attacks, and may even be part of the design.