DOTA 2. The spectator app lets you watch any recent or live matches, plus all of the upcoming matches going down as part of The International tournament.
You'll be able to check out DOTA 2 matches from the perspective of players, or via a directed view of the match, complete with commentary. Eventually the spectator client will be rolled into the final game client ,once DOTA 2 officially launches. The spectator client is a standalone app for the time being.