Trick out your dock with Dock Library
With the advent of Mac OS X Leopard, people seem to be taking the initiative to "hack" their dock and change the image that defines what it looks like. If you don't want to pay money for an application like Panic's CandyBar, then look no further than Dock Library. For freeware, Dock Library has a very simple, clean interface, it's almost Apple-like in it's appearance. Dock Library allows you to import dock styles downloaded from sites like LeopardDocks. Just click a dock style, click set dock, type your password, and your old dock will be replaced with the new one.
Dock Library is available from the developer's website as a free download (donations are accepted).