I have been beta testing Joost. This is a computer app created by the same guys who did Kazaa and Skype. It is a peer-to-peer video broadcasting application and it wants to do to the TV industry what Skype is doing to telephony and Kazaa did to file distribution.
The video and sound quality are very good and the interface is slick, especially considering that this is pre-release software. The downside is that there is still very little to watch, but that should change with time. Also, as far as I can tell, it does not broadcast live events at all (I would pay for certain games, for instance).
Joost is still by invitation only. I have one invite left in my account. If you have an Intel Mac and would like to try it out, leave a comment.