Valve Reveals Steam Broadcasting

Watch friends play, with the click of a button
Now available in beta, Steam Broadcasting allows Steam users to watch their friends' games by clicking on "Watch Game" on their Friend's profile or Friend's List to open a window into their gameplay. No game ownership, special fees, or additional apps are required. The beta is open to everyone who opts in to the Steam Client Beta (via the Steam Settings panel) - however bandwidth may be limited during the beta.