Webcam-Streamer is a free server to stream pictures from a webcam on the Internet. It is available under the GPL.
The GUI of the Program is based on Qt and the images are caught using OpenCV 2.0. It does not use any OS-specific API so it runs on Linux, Mac OS and (with a whole lot of luck) Windows.
The Webcam-Streamer works as an HTTP-Server. If any client sends a GET-request to it, it returns the current image from the webcam. To see this as a running stream, I've written a small Java Applet (Downloads). The address and the port of the server have to be set in the HTML-File as parameters for the Applet.