There are lots of really cool apps out there for X.
AquaMon, that shows detailed system information on the desktop.
DragThing, so your dock isn't unbearably cluttered.
WeatherPop, to display the local weather in the menubar.
TinkerTool, so you can modify OS X to your heart's content.
TransparentDock, so you can make the dock transparent (duh).
I'm sure that there are lots more, but I really need to get to bed