Back in the day, there was really only one way to bring a live video feed from a real world event into Second Life: Quicktime. Using Quicktime Broadcaster, you streamed the live video from a camera connected to a computer in the room to a streaming service. On land where you could control the media streams, you put in the URL of the stream, and set a prim to act as the video screen. In theory, anyone with a media-enabled SL client and a recent version of Quicktime could view your live video feed from your sim.
This method still works, but takes a good amount of technical knowledge to setup and operate.
Nowadays, there are many options for streaming live video to the internet: Stickam, Justin.tv, uStream and Adobe Connect, to name a few popular ones. Some are free and others charge per session, by bandwidth or by number of viewers. Most of these are browser-based, so that anyone with a decent internet connection can watch your event. Some have mobile clients as well so that people on smart phones and tablets can also participate. Several of these also offer text chat, private messaging, and even multiple cameras for multiple participants.
Using embedded media, these web-based video streaming services can be brought into Second Life. The advantage of this solution is that if someone can not access your video in-world, you can just direct them to the web URL to watch it there. And those not in SL can participate without having to create an SL account or learn how to operate their avatar.
So what do you use to bring in a live video feed? Feel free to answer the poll or comment.