Steam's In-Home Streaming feature, previously only available in beta form, has now been released to the general public. The service allows you to stream gameplay from one device onto another, effectively allowing you to play your high-end games on lower quality hardware.
For example, you could use your monster gaming PC to stream gameplay onto a cheap laptop, allowing you to play whatever you like from the comfort of your living room. Pretty neat. Currently you need a PC running Windows, but Valve plan to add Linux, SteamOS and Mac functionality in the future.
There's more information here, as well as a support page if you're having trouble setting it up. You'll need to log into Steam on both devices, which must also be connected to the same home network. Then you'll see a drop-down arrow in your Steam library next to where the Play button is. Click on Stream on the device you want to play on and you're in business.
Tried it out yet? What do you think? It was pretty cool playing Black Flag on my pitiful, battered laptop.