Cover flow, Firewire sharing on wifi, and Spaces.
Spaces, as shown above, allows you to allocate workspaces according to your liking. Leopard is the smoothest operating system I have upgraded so far. And being the mac geek in pink, I was able to port parallel and start windows, and other Mac applications within 2hrs of install. Mac OS 10.5 (leopard) took about 40mins or less to install including reformating (clean install).
I love coverflow. New screensavers. Ability to share your firewire hard disk over WIFI!!! how awesome is that.