Ustream, a great streaming website (founded by a Catholic, nonetheless!), is the de-facto standard for live streaming online. Ustream has made it extremely easy for anyone with a webcam (or a whole dedicated multicam rig) to stream an event from anywhere to thousands of users. Ustream is completely free, and uses Flash for stream delivery (although alternative methods are in the works... and you can stream straight from an iPhone!).

Ustream also offers Watershed, a premium service for larger events, higher quality streams, and more stream customization.

More resources for Ustream live streaming: