Valve have launched their own livestreaming service via Steam, and you can test it out right now if you're signed up for the beta versions of the program.
The service allows you to stream your own gameplay and also watch other players stream their gameplay too, via the new 'Broadcasts' tab on the Steam Community.
If your friends are broadcasting their gameplay you can simply right-click their name in your Friends List and choose to 'Watch Game'. Players can chat with each-other in the chat window at the side of the stream too, just like on Twitch.
Valve's news update says: "The beta is open to everyone on Steam who wants to participate by opting in to the Steam Client beta through the Steam Settings panel. For the time being, concurrent viewing may be limited as the beta is scaled up to support the broader audience."
Check out the Steam Broadcasting update page for more info.