Since Mac OS X 10.5 "Leopard" Apple has introduced a new kind of Dock with two different states: 2D standard view and 3D-Like View. Each of these views can be modified by replacing some resources inside Dock.app itself. PimpMyDock allows you to create and modify easily your Dock appareance by using simple .docktheme files. Each docktheme file contains all resources; just drag it in PimpMyDock and click Apply Theme: PimpMyDocks will do all the job for you! Easily and secure!
If you want you can create yourself a theme, or importing a one from http://leoparddocks.com/ (PMD allows you to import data from folder and creates automatically a docktheme file). You need to use Generate Theme From Folder option in order to transform automatically these resources file in a docktheme; take a look to quickstart guide.