Valve have released a Spectator Client for the upcoming Dota 2 on Steam. It grants users access to the many spectator features of the game, that among other things allow users to view tournament games with commentary, both live and after the fact.
The Spectator Client is a free download, and will give you access to the upcoming million dollar tournament The International that takes place during PAX Prime.
If you have Steam installed you hit this link to download the client.
Dota 2 is currently in closed beta, but you can buy in for a royal fee of $25, while the final version of the game will be free-to-play.