iTunes Library Updater
If your iTunes library is disheveled and unorganized after moving or deleting songs, grab this handy free tool. It can clean up duplicates and orphaned entries, as well as add new music from specified directories that iTunes doesn't yet know about.
The author encourages backing up the iTunes databases before using ITLU:
The files are called "iTunes Music Library.xml" and "iTunes Library.itl" and are located under "My Music\iTunes\". Copy them to another location. This allows you to go back if anything does not work as expected.