id3j is a lightweight and easy-to-use library to create and add ID3 metadata tags to MP3 files. It only concerns itself with ID3 version 3.2.0 and only handles the most common types of information, but creating a tag and adding it to a file is much easier than with many other solutions. It also has one fairly esoteric feature that is interesting from a algorithmical standpoint, which is the ability to create CRC32 neutral ID3 tags. That means that you can add such a tag to any MP3 file and the CRC32 sum of the resulting MP3 file with the tag will be the same as the file without the tag.
MP3 Diags finds problems in MP3 files and helps the user fix many of them. It looks at both the audio part (VBR info, quality, normalization) and the tags containing track information (ID3). It has a tag editor, which can download album information (including cover art) from MusicBrainz and Discogs, as well as paste data from the clipboard. Track information can also be extracted from a file's name. Another component is the file renamer, which can rename files based on the fields in their ID3V2 tag (artist, track number, album, genre, etc.).
SQL Tagger reads the tags of audio files from a directory, loads them into a temporary SQLite database, accepts SQL queries to modify this database, then saves any changes back to the audio files. It's useful for performing complex updates that would be difficult or tedious with conventional taggers. It's a complement to conventional taggers, not a replacement.
id321 is a command line ID3 tag manipulation tool. It supports reading and writing of ID3 tags of the following versions: ID3v1, ID3v1.1, ID3v1.2, ID3v1.3, enhanced tag, ID3v2.2, ID3v2.3, and ID3v2.4. The utility can force standards of ID3 tags in order to fix existing non-standard tags.