The PC users with nVIDIA graphics accelerator are in luck. If you have a GPU of the GTX 600/700 series, you can send your games and play them streaming through the Raspberry Pi...
Limelight is an open source Java implementation of GameStream nVIDIA, but built for Linux/OSX/Windows.
Limelight Pi is still in a very early stage, but will transmit your Steam games to your TV through the network with the Pi.
Maybe is not a Steam Machine killer, but comes close.
Updated: A user has tried and it works!. He recommended that we install to solve input and audio problems the library libopus-dev. Thanks Felix Piechaczyk!.
Download: Limelight-pi v0.1 (Alpha)