Sites like omegle.com want video from my connected camera, but I don't have one. How can I feed omegle.com (a Flash app) a spoofed video feed (eg, a static picture, mp4 file, etc?)
CamTwist creates "pseudo-video" devices for lots of different things, so you could display your desktop or any subset of it, or other content you choose. CamTwist's feature set is likely very similar to Ryan Sharp's suggestion for ManyCam.
iGlasses is one way to do this. It shows up as a camera on websites, and you can play a video or show an image by dragging it into the effects tab.