Just announced at this year's GDC, OnLive is an on-demand gaming service. It's essentially the gaming version of cloud computing - everything is computed, rendered and housed online. In its simplest description, your controller inputs are uploaded, a high-end server takes your inputs and plays the game, and then a video stream of the output is sent back to your computer. Think of it as something like Youtube or Hulu for games.
When you load up the service and choose a game to play, it starts immediately. The game is housed and played on one of OnLive's servers, so there's never anything to download. Using an appropriate input device, be it a controller or mouse and keyboard, you'll then play the game as you would if it were installed on your local machine. The server then plays the game just like it would if you were sitting at the machine, except that instead of outputting the video to a display, it gets compressed and streamed to your computer where you can see the action. Rinse and repeat 60 times per second.
The cool thing here is that your only requirement is a capable internet connection and some sort of computer. In theory, you should be able to play Crysis on a netbook.
A handful of people have played the game, at its highest settings, on a MacBook Air with the service
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Source:
http://pc.ign.com/articles/965/965535p1.html
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Official website:
http://www.onlive.com/