Some of my favorite co-op memories involve the Sega Genesis and games of NHL 95 with my buddy.  Sports games are a natural co-op occurence and most games will let you play locally with at least two players vs the AI.  Sure we've had a few games that have added a dedicated online co-op mode, but these modes so far haven't been persistent.  All of that looks to change with NHL 13's new mode called GM Connected.