How can I view a live Starcraft II match in-game?

When I watch screencasts of Starcraft II, I notice that there are people leaving the game at the end that weren't part of the match. How do I spectate a Starcraft II match?


Solution 1:

I believe that currently the only way is to get invited (as in, there isn't a list of available games to watch somewhere)

Obviously ladder games cannot be watched, but you can watch custom games if it's set to allow watchers, and the creator adds you to them.