I must begin by giving the proper credit where it is due: I found this hint by way of GUI gurus demograph68 and Dave-o from the MacNN forums. Now then, if you're tired of having a plain white sidebar in your Finder windows, you can change the color to any you wish.
Note: This procedure involves modifying files crucial to the OS and, if done incorrectly, can basically hose your Finder!. So, back up the files to be changed before you change them. Also note that this procedure works only for 10.3.5, and may break with future system updates. Read the rest of the hint for the detailed how-to...
[robg adds: I tested this one, and it indeed works, as you can see in the image at left (now I just need to go pick a color I really want to use!). So any hotshot Cocoa authors out there want to create a GUI for this hack? Not knowing the first thing about Cocoa, I have no idea if it's even possible to modify the Finder on the fly, nor if it's "safe" to do so ... but it'd be a cool hack!]
Navigate to System -> Library -> CoreServices -> Finder -> Contents -> MacOS -> Finder. You will need to Control-click and select Show Package Contents on the Finder item to get to the Contents folder). Now copy the Finder file to a safe place and keep it as your backup. Make another copy of the Finder on your desktop, and open this Finder copy in HexEdit, press Command-J, and type in 0029EB10 then click GO. You will see this:

Mac OS X Hints
http://hints.macworld.com/article.php?story=20040923032352321