As a graphic and web designer I spend most of my time on one of my two computers. The deciding factor as to what station I use, is based on whether I want to sit at a desk and use my desktop, or sit on the sofa/lie in bed and use my laptop. More times than I care to admit the laptop/sofa combination wins.
But there is one thing that is the same regardless of what computer I am working on, I want it to look inspiring and reflect my current style - whatever I am feeling at the moment and there is no quicker way to do that then to change the desktop/theme.
Pixelgirl Presents is a site by Morfia Design and is full of Mac OS X and XP desktop images, icons for your application sets and chatting needs - plus they offer tutorials, articles, links and a gallery shop full of handmade goodies.
The desktop images can be searched by category (cute, photographic, tech, urban to name a few), size or artist. They are free to download, so head over there to submit your own or snag the creation of someone else.
You and your computer will thank me!
Here are the desktops currently on rotation on my Powerbook:

Get your own:: Pixelgirl Presents Desktop Images