ID Infiltr8 is my tagging program for mp3 & m4a files. I got tired of having to manually capitalize & tag my audio files. It was so incredibly repetetive - so I started out on a program that could do it for me (ala EasyTag for Linux). That program was called mp3Spec and was limited to setting a few tags & file renaming.
This is the program I've wanted to write for a long time now, & it's now fully functional. The last big part was writing to m4a files - that now happens perfectly with AtomicParsley. AtomicParsley is my own command line mpeg4 tagging tool that supports multiple pieces of artwork & lyrics - along with the image resizing/limiting features introduced into id3Gateway and is opensource & GPL'ed at sourceforge.