Possibly the most comprehensive discography site on the web there are crowdsourced listings for millions of releases, along with a trading community. WIth its origins in DJ culture, it is an incredibly valuable reference source for tracking down obscure dance music - or different regional versions of particular releases. For some reason I end up on Discogs every day - either to answer some question about a particular track, or to use it as a demonstration of the power of well structured, crowdsourced metadata - something that museums could learn a lot from.