The "Rate the importance of multiplayer" thread got me thinking about one game that really shocked me with it's muliplayer. That game was the Xbox Live Arcade version of Outrun 2. Outrun has always been primarily a single player game so it seemed rather weird to me that there would even be a multiplayer game, however there was and it was pretty cool. There wasn't really anything particularly noteworthy about it, at times the game was glicthy as hell, but it did have a sizable player base and I enjoyed racing people online almost as much as the single player.
So what about you? What are the multiplayer modes that really surprised you?