I was watching a Twitch.TV stream of a panel of announcers discussing eSports live, and while all of the graphics, sets, backgrounds, lighting looked great, I thought it would be nice if the raw video of the announcers was also processed with composite editing, for a surreal, polished film effect. But I'm not sure: Can a video be composited fast enough to be streamed live, and are there tools capable of doing this?
The answer is yes, video can be composited fast enough to be streamed live. The tool that does this is called a switcher. Typically a switcher introduces a single frame delay from input to composited output. By connecting the output to a streaming encoder, you can stream out your live, composited video.
For an example, see the Blackmagic Design ATEM System.