MultiTunes is an iTunes plug-in that lets you manage multiple iTunes libraries simply and seamlessly from within iTunes itself. There is no external program to run. It will work on Intel or PowerPC Macintoshes running at least Mac OS X version 10.3 (including Leopard).
MultiTunes does not move any of your music files when you switch libraries. In fact, only 3 files are touched: "iTunes Library" and "iTunes Music Library.xml" from the "Home -> Music -> iTunes" directory, and "com.apple.iTunes.plist" from "Home -> Library -> Preferences". These 3 files are moved in and out of the holding directories in "Home -> Library -> Application Support -> MultiTunes".
Even though your music files are not touched by MultiTunes, you can still have multiple music libraries on different disks. After creating a new Library go to the "Advanced" tab in the iTunes preferences where you can change where the music files for that library are stored. Note that even if you change where your music files are stored, the 3 files that MultiTunes touches still remain in their normal place.
MultiTunes is shareware. You can try it for a while to make sure it works for you. Then you can purchase a registration code for $15.99.
If you used version 1.3 when iTunes 8 came out your iTunes may have started giving you an error like this:
And that should fix it. Don't worry, the files will be recreated when you run iTunes next.