The Steam patch mentioned earlier adds the FPS Counter previously in beta as well as Steam Broadcasting, which is just what it sounds like, a Twitch-like service to broadcast and view game sessions. This FAQ answers questions about this, including the following:

How do I watch a friend's game?
From the Friend's List - From the drop down next to a friend's name, select "Watch Game".
From a friend's profile - Below the "Currently In Game" text, select "Watch Game".

Who can watch my games?
The first time a friend requests to watch your game you'll be presented with the broadcast privacy settings. To make a change at any time, access the Steam Client's Settings panel and select the Broadcast tab to view your privacy settings.

Available settings:

• Broadcast is disabled
• Friends can request to watch my games (default)
• Friends can watch my games
• Anyone can watch my games (public broadcast, can be found in the game hub)