Cairo Dock works with or without OpenGL, and makes for a surprisingly functional visual improvement over the spartan Ubuntu Classic or Unity 2D interfaces in Ubuntu. It doesn’t just look slick, though; the dock allows customisation for behaviour, appearance, plug-ins and themes.
The OpenGL version of Cairo Dock is also known as GLX-Dock, and the two share all plug-ins as well as themes.
Both the dock itself and the app icons that sit on it can have their own separate themes, and you can add not just multiple docks but sub-docks for existing docks. More interestingly, you can add applets alongside apps, which is something the dock in OS X simply won’t do.