As the Steam Developer Days conference kicked off in Seattle, Valve sent out invitations for a new Steam service: in-home streaming. That's pretty much exactly what it sounds like, folks; once the service is out of beta, users who've gained access to the beta will be able to play Steam games in their library on other systems - even if the games aren't actually installed on said system. Now there's a minor caveat to that: at the moment, this service is only accessible if the system you're using is on the same home network. You will not, for example, be able to head over to a friend's house in order to access the service. External streaming is not currently supported (and to my knowledge, there aren't plans to support it).
In addition, whatever system the game's being streamed from will not be usable, as it'll be running the game. Basically, you'll be controlling your game via remote access, meaning you could feasibly stream a game to a computer which might otherwise be unable to run it. Pretty cool right?
As for the "in home" limitation, one could feasibly use a third-party application such as Hamachi to fool Steam into thinking two systems were on the same local network.
Now, although Valve's claimed it's only sent out beta invitations to development partners, a number of general steam members appear to have gained access to the program as well, and took to both Twitter and the streaming service's community page to share their impressions. Although a few users have experienced graphical glitches, poor performance, and black screens, as a whole reception for the program has been overwhelmingly positive.
Anyway, those of you who still want to sign up and give the service a try can join the official Steam In-Home Streaming group. I'm not sure what further plans Valve has for In-Home streaming, but I'm certain they'll be exciting, whatever they are.