Today, Valve quietly launched a new livestreaming service called Steam.tv. The site went live earlier today, though at the time of this writing was inaccessible.
Steam.tv looks to be primarily a livestreaming platform, though at launch is only showing Dota 2's The International. Though GamesIndustry.biz was unable to access the site consistently enough to test its features, CNET reports that the interface includes Steam Chat functionality with tabbed chat windows and the ability to invite friends to watch videos together. Google Chrome currently supports in-browser voice chat as well.
Before this, Steam users were already able to stream their gameplay through the normal Steam application, but it was only accessible through the app to those already connected to the user. Steam.tv appears to be more along the lines of a streaming site competitive with Twitch or YouTube Gaming.