Using your text editor of choice, edit this strings file: System -> Library -> CoreServices -> SystemFolderLocalizations -> en.lproj -> SystemFolderLocalizations.strings. If you use Mac OS X in a language other than English, edit the file in the directory for your language.
Suppose you want to rename Library to Stuff. You'll want to change this line:
"Library" = "Library";to read
"Library" = "Stuff";At the top level, you may rename System, Applications, Library, Users, Shared, and Utilities. In your home directory, you can rename Desktop, Documents, Movies, Music, Pictures, Public, Sites, and Drop Box. Log out and log back in and you should see the directories renamed. This change does not actually rename anything in the file structure, so if you browse your files in the Terminal you won't see any changes (and software which expects certain names will continue to work).
Note: In my experience on 10.2.1, the Finder localizes the top-level folders but not folders in home directories. Comment on your experiences.
[Editor's note: As this file is owned by the system, you'll either need to edit it in the terminal with root privileges, or use the Get Info window to temporarily change ownership of the "en.lproj" folder to yourself, edit the file in the Finder and accept the overwrite warning when you try to save, and the change the directory ownership back to system.]

