Windows 7's new Libraries are one of the best underhyped features because they make it easy to manage files across multiple folders — the only problem is that all of your libraries use the same bland icon.
To explain further, if you've created a number of your own custom Libraries to store your files, you'll notice that all the icons look the same, which is especially annoying for the overly-broad "General" folder type. The solution is a simple little app called Replace Libraries Icons which allows you to easily set the icons for each Library to whatever you would prefer them to be.
Just open up the no-installation-required application, select the Library you want to change on the right, then pick an icon from a .DLL file or from an .ICO file, and the changes should be made instantly. The application also allows you to revert the icons back to what they used to be, so if your icon-changing experiment doesn't work out, you can get back to normal.
All the utility actually does is provide an easy interface to edit the XML file that represents each library — so if you are more of a hands-on type or want to understand how it works, I've also written up a quick guide on how to edit the library icons manually using Notepad (or your favourite text editor). Replace Libraries Icons is a free download for Windows only.