Originally Posted by nabl

Like Collin said, you can use iChat, which is installed by default on your Mac. It's capable of all sorts of effects in OS X versions since Leopard. There's a video here on Apple's website that shows some basics. To get effects, just click the Effects button at the bottom left of video chat windows. On the last page of effects, you can drag any image from your computer to a blank effect to use it as a video chat background.

Oh, and if you aren't in need of using this during chat (like if you're just recording videos), open the Photo Booth application from your Applications folder. It's capable of all the same effects as iChat, and it's just as easy to use (perhaps easier).