Valve is now letting its millions of Steam users stream their games to the world. Steam Broadcasting, the company's answer to the wildly popular Twitch, was announced back in December and has been included in beta clients since, but today Valve has added the feature to the regular Steam app.
Broadcasting and watching gaming is pretty simple. If you know someone's Steam ID, you only need browse to their profile and click "Watch Game." This'll send a request through to the player's Steam client, and if they accept it'll start streaming immediately. You can change the privacy settings to, say, keep streaming totally public, limit it to friends or make it invite-only. Public streams are discoverable through the Broadcast option under the Community tab.