The developers who were at the WWDC received beta copies of OS X Leopard, so they could start developing for it. Some developers were good, and kept it to themselves. Others released screenshots that they weren’t supposed to. Others actually released the OS itself onto the internet.
This guy decided to post some videos of it in action. You can see Spaces, Time Machine, Stacks, the new screensavers (cool stuff), and a lot more.