You must already be using something outside the client-side flash, or else users would not be able to see anything from other users. If you prefer to keep everything open source and on your own servers, then you could use red5's rtmp processing stuff to handle video.

http://code.google.com/p/red5/

Why don't you describe what you're already doing? Here's my guess: every so often, you capture webcam video into a bitmap. You then JPG encode that bitmap (or PNG depending on whether you are trading time for accuracy), Base64 encode that, and send it to your server via an xmlsocket. Through the same or a different socket, your server passes that base64 encoded image to all the relevant clients who decode it on their sides and display it.