TagEncoder is an application that tries to take the pain out of converting a large number of MP3s from one character set to another. Specifically, it is to help people converting MP3s with ID3 tags encoded in a character set that iTunes doesn't understand.
All you need to do is drag and drop MP3s from either iTunes or Finder to TagEncoder's window. TagEncoder will attempt to convert the tags using a list of character sets that you ask it to guess with. Once you are satisfied with results, you can drag the songs directly into iTunes or anywhere else and new files will be created.
It will never overwrite your original files and it will preserve all the other hidden ID3 tags in your MP3s.