Windows: Libraries are one of the most useful features in Windows 7, but are a bit restricted in what they can do. Free utility Win7 Library Tool gives you everything you need to fine-tune, customise and extend your library collections.
The only thing you can really do with Windows 7's default library editor is pick indexed folders to include in a specified library. Win7 Library Tool shows you all your libraries in one window, and when you choose to edit one, you can edit tons of otherwise hidden Library properties. You can change a library's icon, create a mirror (allowing you to make the path to a library much shorter), add network or other non-indexed folders to your libraries, and even back them up for restoration later on (say, if you decide to do a clean install). Essentially, it's what library management in Windows 7 should have been.
Win7 Library tool is a free download for Windows 7 only. If you've got a great use case for extended Windows 7 libraries, let's hear about 'em in the comments.