If you're bored of the dull, grey dock that OS X ships with, Dock Designer will help you quickly zazz it up into something more fun.
You may have seen sites like Leopard Docks before, which despite the name, offer customised docks for OS X Leopard, Snow Leopard and Lion. The manual installation process is a bit tricky, though, and a little long. Dock Designer fixes all of that. It doesn't do a lot, but it will let you choose a new image for your dock, choose new dock indicators, switch between 2D and 3D and add transparent separators to either side. And, if you don't like your new look, you can reset it to the default with just a few clicks.
Dock Designer is a free download for OS X, most features are 10.7 Lion only.