Games for Windows - Live
Full user review
"Downloads game demos kinda like XBox Live Marketplace."
It uses the Microsoft Points system, and Gamerscore as well.
Game demos will have to be of crap quality on some computers. I have an HP laptop with an AMD Dual-Core Processor, 4 GB of RAM, and a 280 GB Hard Drive, and it delivers decent quality gaming. WEI rated this a 4 out of 8.
If you click Game Marketplace on your Xbox, it does the same thing.