Justin.tv was the pioneer, a free platform that after registration you can open your own channel (so a web tv) and start broadcast video in streaming, if you want to start broadcast yourself here is a list of valid alternatives you should consider:
Ustream
add viewer chat rooms next to your live broadcasts, embed your live/recorded shows on your blog or profile, browse shows created by others by various criteria
Veetle
a Windows application must be installed to use it, you can watch it also on iPhone/iPad, widget for own web site
Livestream (ex Mogolus)
provides the “studio”, an elaborate flash app on their website where you can do all the jobs. Each channel also includes a library area where you can add videos from files, YouTube videos etc…; video player can be enbedded for own site/blog or also there is a Facebook app, Twitter integration, free Procaster software to broadcast from your desktop, you can choose between free or paid plans (premium)