I think they'll offer LBP, inFamous, MAG, Warhawk, and Mass Effect 2. Can you download Modnation Racers?
Mass Effect 2 is exceedingly unlikely, and Warhawk is fairly unlikely too. Neither of those are first party titles. The free games will most likely be owned entirely by Sony so they don't as such lose money by giving them away. Otherwise they would need to pay the developers/owners.