Step 1) Find a web-cam video of a celebrity on youtube.com
Step 2) Download the video with Keep Vid
Step 3) Convert the video to MPG with Pazera
Step 4) Use Virtual Dub to cut out a looping section of the video
Step 5) Download Many Cam and set it up with that video you made. (It will make a virtual web-camera with that video looping in it)
Step 6) Head over to Chat Roulette and point it to your many cam.
Step 7) Watch the hilarity of people's reactions. Priceless
Most people figure out pretty quick that its fake. Like when the video doesn't react to them and especially if the loop isn't perfect. Still lots of fun while your waiting for SC2 to install.... and it does take a looooong time to install.
PS. I'm still looking for a good Arnold Schwarzenegger and Chuck Norris video. Comment with your favorite source videos. A good example one is Jessica Alba.
Other probably funny ones:
Woody from Toy Story 3
Gatekeeper? No clue, but its funny