puddletag
SImple, powerful audio tagger for GNU/Linux
...Then there’re Functions, which can do things like replace text, trim it, do case conversions, etc. Actions can automate repetitive tasks. Doing web lookups using Amazon (including cover art), Discogs (does cover art too!), FreeDB and MusicBrainz is also supported. There’s quite a bit more, but I’ve reached my comma quota.
Supported formats: ID3v1, ID3v2 (mp3), MP4 (mp4, m4a, etc.), VorbisComments (ogg, flac), Musepack (mpc), Monkey’s Audio (.ape) and WavPack (wv).