MP3 - Features history and technical information about the MP3 format. MPEG Audio - Theo Michel presents an introduction to the MP3 format. Includes discussions of legal and ethical issues and other resources. MPEG Audio Frame Header - Technical explanation of how MPEG audio files and their headers are constructed. MPEG Facts and Info - Number of frequently asked questions about MPEG audio compression and other technical information.
Site contains many implementation details as well as photos.
R3mix.net - Comprehensive source of information about MP3 ripping and encoding. Reports on quality issues at various bitrates and common myths about sound recordings. XHAudio.net - Source of MP3 software downloads and consumer information about portable players.