In preparation for The International 2, the second annual Valve sponsored Dota 2 tournament, Valve is allowing each and every Steam user to download the Dota 2 spectating client to view the tournament in-game. Valve insists that this is the best way to watch any Dota 2 tournament, due to a number of various features not otherwise possible.
This spectator client will transition into the regular Dota 2 client, with all of the features, when the game is fully released at a later time.
The International 2 takes place August 31st through September 2nd in Seattle Washington, and if your computer can't quite run Dota 2 it will still be viewable via live streams. For more information check out the Dota 2 Blog.