Defense of the Ancients 2 players in the world duking it out, or enjoy replays of matches, Valve has released the spectator client for DOTA2.
Players can then watch current and recent casual matches for free, though most tournaments charge a small entry fee to view them. The Wild Card rounds from The International, however, are free, featuring some of the best players in the world. In addition, players also have a choice of multiple commentators for matches as well.
The spectator client will be upgraded in the future into a regular DOTA2 client, according to Valve, "once we expand server capacity."
The spectator client can be downloaded from Steam.