Change the OS X Dock background

LeopardDocks is a great website that has lots and lots of different replacement dock backgrounds. To use the files the website provides, just use their LeopardDocks App.

I've also had problems with the default Leopard Dock. If you use the following commands in the terminal, you can get a different, 2-dimensional dock:

coputername:~ user$ defaults write com.apple.dock no-glass -boolean YES
coputername:~ user$ killall Dock

If you change your mind, you can restore the 3-dimensional dock with the following commands:

coputername:~ user$ defaults write com.apple.dock no-glass -boolean NO
coputername:~ user$ killall Dock

I think the best combo is Candybar and various docks from MacThemes or the Iconfactory. Candybar is simple, quick, and effective.

You might want to look at this question about tweaking the UI in OS X.