Best sound quality in compressed audio: interesting page concerning professional AAC and how it compares to other encoding schemes.

Fraunhofer: Fraunhofer Institute's audio section. Basic technical documentation.

ID3v2: The official website of the new MP3 tagging specification. It provides both the formal and informal specification documents of ID3V2.

Mp3.com: Daily updated mp3 news and free music download.

Mpeg.org: Very complete directory of links to mpeg resources on the net.

VQF.com: Site about the VQF file format.

The Definitive MP3 Benchmark: Good site benchmarking MP3 players and encoders. There are few technical explanations too.