onLive will allow you to play games like
Assassin's Creed 2
Batman: Arkham Asylum
Mass Effect 2
NBA 2K10
Prince of Persia
and more
This service is available for Mac and Windows. The game client will connect to a server and will allow you to play the game. This need to have a good internet connection. The service costs around 5$/month but they are giving a special offer now "OnLive Founding Members Program", if you register now they will give you (Subject to the availability) an 1 year account for free
here is the promotional page, enter the details and wait for the mail from them
Read more here
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[Via WebiSee]