Searching for a "MP3tag"-style tagger for Mac, I found "Mac3tag".
It's written in Java by Igor Rudman and only supports MP3 so far, but I hope it will support more formats in the future as I kinda like it.

http://sourceforge.net/projects/mac3tag/

Magnus

Nice suggestion, thx I downloaded it and im going to test it these days, but judging by screenshots, it looks very basic compared to the wonderful Tagscanner 5.1 on Windows. Im still using Windows mainly coz of Tagscanner