Super Stardust™ HD
Bought this a while ago and have put *many* hours into it. If you like old-style arcade games (your progress isn't saved, so you start over every time) and trying for high scores then this is a must-have.
LittleBigPlanet™
I own this on disc, and it's a very novel game. The mood the game sets is actually very engrossing, but can also be a bit depressing for some reason. If you like games that go in completely new directions, and you like platforming, then this is a great game.
InFAMOUS
I chose this as my first free one as it was the no-brainer choice. It's a great game and I've put a ton of hours into it over the last couple weeks. It is not a superb game, just a really solid game that's lots of fun.
WipEout® HD Bundle
I grew up playing F-Zero so I chose this over Dead Nation for my second free game. Dead Nation is a dual-stick shooter made by the people that made Super Stardust HD, and I love that game, so this was a hard decision. Comparing prices if I was to buy them (as Aikouka did above) was what swung me over to this one. WipEout HD has actual advertisements in game though, which if I had known about I would *not* have chosen this. Every time a damn commercial plays I want to quit the game and not play it again.
Dead Nation
This looks like a lot of fun and I will probably pay the cash for this game at some point in the future. I wish I'd chosen it over WipEout HD.