Of course, iTunes serves as a fine tag editor. About the only features it's missing are any way to set the tag from the file name or vice versa. iTunes, however, can happily scan your files in place, without importing them anywhere, and let you edit the ID3 tags within.

You can also use any of the Unix tag editors, like kid3 (requiring you to install Apple's X server and probably a big chunk of KDE stuff from fink).