MP3 Diags
While a good part of the code is rather complicated and could use a refactoring (or at least break up some large files), it's quite easy for a programmer to change MP3 Diags to detect new issues or to add new tools to fix these issues, because these things are handled through simple interfaces. I'll provide details if anybody wants them.
Should you want to make any changes to the code, it's better to contact me first, especially if you expect those changes to be merged into the official code. Even if you don't want your changes to be included, I might have some suggestions about how they should be implemented.