With The International coming up (DOTA2 tourney), Valve is stepping it up a notch and wants everyone to watch The Tourney and Live DOTA games for free!
http://store.steampowered.com/app/570/
If you fail to understand, here's a few explanations on what the Spectator client is and its features:
http://www.reddit.com/r/leagueoflegends/comments/ye9er/valve_giving_free_dota_2_spectator_client_to/c5urzfh
The best part is when DOTA2 comes out of beta, your spectator client will automatically update to the latest patch, meaning you won't have to deal with the hassle of downloading everything (about 1-4GB) from scratch, only the latest updates (50-200MB, usually).