You can change the source on
WebcamMax to show your screen and it should work with anything that is able to broadcast a webcam (MSN, Yahoo, Stickam, Camfrog, etc.) It has a banner accross the top or bottom of the video feed with the WebcamMax logo until you register or crack it.
Windows Media Encoder 9 has this feature too I think, I've never tried it so I can't say how easy it is. However, I have been able to use it to make a live windows media stream of my webcam before so you could probably use it in conjuction with WebcamMax to make a live media stream showing your screen.
Also, I don't know how either of these work broadcasting video overlays or 3d games and stuff.
Jam it back in, in the dark.