At first this caught my eye:
http://www.gamefly.com/download-games/GameFlyClient/
Ohh, unlimited pc play, with a screenshot of hot titles. Went to the list. A bunch of shit. Talk about misleading.
You use the Gamefly client. Use the same account you purchased the games with and they'll be there for download/install. Bioshock 1 doesn't need the client after install, but 2 does.
I wasn't sure at first if it was a standalone download, but GameFly has a download client that also integrates with their rental service.
Edit: I'm actually a little impressed by their Unlimited Play category. I can't find for sure if you have to purchase a game first or anything, but I think it's just free for anyone who registers and downloads their client.