I've been using this one quite a lot lately because of it's ease of use. It's a little buggy, but I find it fast and convenient and I can work around the bugs.
It's called MP3ext. It basically adds another property page in explorer for MP3 files. Basically, bring up the properties on a file and you can edit it's ID3v2 tag. For backward compatibility, it even puts an ID3v1 tag on the file as well (truncating strings where necessary).
At first I found it clumsy, but once I learned how everything worked it all made sense.
Anyway, here is where it is:
www.mutschler.de/mp3extKureg