Cloud Gaming is a relatively new site covering the range of thin-client style solutions for gaming. We discuss this briefly in Chapter 13, and mention OnLive, but since the book was written (i.e. since summer 2009) several more services have started. RealityServer targets more general graphics applications, whilst OnLive, Gakai & Playcast are targetting games. OTOY is combining both realms.
We'll write an analysis of the requirements in the next few months. Services are most likely to require dedicated hardware, plugins or flash video. Of course something can always be done with pure HTML: see this demo of NOVA Server.