CamTwist Studio is a free tool that helps you turn your Mac into a television. It comes with lots of features giving you the opportunity to create your own effects based on a library with amazing out of the box idea. It works with HDMI Capture Cards, with a High Definition resolution and it can stream live to the web using a common provider, like Justin.tv, Livestream or BlogTV.
Obviously, the most important thing is the fact that is 100% free, and you can download it in minutes.
On CamTwist Studio website, you can learn, step by step, how to use it properly.