TagLib is a library for reading and editing the metadata of several popular audio formats. Currently, it supports both ID3v1 and ID3v2 for MP3 files, Ogg Vorbis comments and ID3 tags in FLAC, MPC, Speex, WavPack, TrueAudio, WAV, AIFF, MP4, APE, ASF, DSF, DFF and AAC files.
TagLib is distributed under the GNU Lesser General Public License (LGPL) and Mozilla Public License (MPL). Essentially that means that it may be used in proprietary applications, but if changes are made to TagLib they must be contributed back to the project. Please review the licenses if you are considering using TagLib in your project.