It is this little blue square that appears at the end of every possible link in any window and you can preview what is behind that link without ever clicking on it. It makes browsing so much easier and my tab bar at the bottom of my screen is still clear.
It is called CoolIris and it is absolutely free. You can stack up your windows, go back, email, etc. There is another really great tool for pictures on the same link called PicLens.