Designed for all those that don't want iTunes managing their music, TriTag is a hugely popular utility for automatically managing the ID3 labels of your MP3 collection.
TriTag doesn't actually allow you to manually edit ID3 tags. However, what it does do is automatically retag ID3 and MP3 file names in batches. TriTag helps you instantly organize your MP3 collection either by creating a new filename out of the ID3 Tag or filling-in the metatags using the information found in the filename. Using the former, you can instantly create unique directory structures (such as artist/album/track), in a similar way to the 'Consolidate Library' function in iTunes.
If neither the filename or ID3 tag contains any info you used to be able to search the FreeDB database in TriTag to tag them. However, freedb.org is now operated my MAGIX and it no longer offers a web-based search interface, so this function doesn't work at the moment. This does rather significantly reduce the usefulness of TriTag but the developers hope it will be fixed soon.
The overall interface of TriTag could do with an overhaul as it's rather plain and ugly but in terms of functionality, when fully working this program is ideal for cleaning-up badly tagged MP3 collections.