Mp3Tag is a sweet little tag editor / renamer for music files. Written by a german company, it basically scans any folder you give it, and provides a huge range of features, including swapping between formats on filename or ID3 tag, batch renaming, and best of all, renaming from a text file.
This last bit is particularly handy if say, you have a folder of mp3s labelled Track 01, track 02 and so on, but without any tags. You could type them all in by hand. OR you could get a tracklist off the old series-of-tubes and stick it in there.
The program can take some getting used to initially, but its a very powerful app.