Update: After a weekend of speculation, Valve has officially launched Steam.tv alongside an update to the Dota 2 blog explaining the feature.
Steam.tv will serve as a hub for a major update to how Steam Broadcasting works, debuting with Dota 2's The International this weekend. The update brings Steamworks support and a Live DVR feature to Steam Broadcasting. This allows for special markers for key moments in the broadcast, as well as an immediately-available rewind feature.
The update also includes Steam Chat integration and the ability to create Watch Parties for broadcasts events. Users can particpate in the main Steam Chat for the event, or create an invited group to chat with while watching.
Watch Party support will be rolled out to more games after The International this weekend alongside a new Steamworks API for Steam partners.
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.
Valve has not yet released an official announcement on its plans for Steam.tv; it was only discovered to have registered the domain name earlier today.