It looks as though we'll all get to play Saints Row: The Third for free this weekend. If you like the idea of blasting through an open city in a VTOL jet that shoots lasers, or calling in a gang of pink ninjas to start a turf war with an army of luchadores then you should certainly check it out, especially if you have a friend to jump in with. Saints Row: The Third is quite excellent in co-op.
As Gamespy note, the free trial is currently only visible in your Steam purchase history. A proper pre-load icon will surely hit Steam libraries later this week.