Ex Falso - MP3 Tag Editor

I've just discovered Ex Falso, a GUI-based audio-file tag editor (MP3s and everything else). You can manually add and edit tags, create tags based on file names, rename files based on tags, add sequential track numbers, all the general things you need. It's part of the Quod Libet media player, but if you're happy with your media player you can use the Ex Falso editor as stand-alone.

It's a GTK+ app, written in Python and is in the standard Ubuntu repositories:

sudo apt-get install exfalso