Mac OS X Tip: Use a solid color background of any color
I've been annoyed by the inability to easily set a solid color background since I started using OS X. Apple ships a couple "Solid background" png files, but they're without fail Not the colors I want.
Turns out, there's a nice, easy way to get a solid color background of your choice that doesn't involve making a new image for each color you want. OS X supports transparency in PNGs used as wallpaper, showing the desktop color through the transparent parts. If you use a small transparent PNG and set it to "Center", you'll get a solid color desktop, and you can change the color using the color picker next to the "Center" pull-down.
I've conveniently attached a 128x128 totally transparent png file below:

Right-click above me!
Save this file to your "Pictures" folder, open up System Preferences, pick Desktop & Screensaver, select the blank picture, and choose "Center" from the orientation pulldown menu. A color selection will appear to the left - click it, and select the color you want your desktop to be.
Simple, but this has been periodically bugging the hell out of me for about a decade now.
Note: This is based on a tip on MacOSXHints, found here:
http://www.macosxhints.com/article.php?story=20021002055217828