If you have at least 300-500kps of upload bandwith available you can push an rtmp stream of your live event to a CDN like youtube, dailymotion, livestream ...
In terms of software I already used:
- Livestream, which provides among other things a tool called procaster with which you can broadcast your desktop, or a section of your desktop. You can even broadcast a capture of your skype window ! ( it will not be very beautiful, but it works ). Livestream also has many tools which allow to make a chat during your live event with the viewers of the event.
- Flash Media Encoder (aka FME ), which is a free tool provided by Adobe, which will encode to rtmp any video camera connected to your computer. You can then push it to a CDN like YouTube or DailyMotion.
All of this is free of charge, with sometimes some limits on the number of simultaneous viewer, and/on an ad-supporting scheme which will play an advertisement before your live video.