I recently (as of yesterday) started playing Saints Row 2 co-op with my friend yesterday again after a long while.  I forgot how fun it is, and all the crazy stuff you can do in it.  Although the pc version you can use mods when playing online with a friend, it was not ported well as is runs too fast in windows 7 and performs badly on my excellent rig.  I would definitely recommend the 360 version for coop play.