This is a clearing house for free recordings of Classical music on the web. Quality varies wildly, as you can imagine, but
it is sometimes worth sifting through for decent, copyright-compliant tracks.
These guys offer both recordings and sheet music for free.
Much of ClassicCat's content comes from here. If you are looking for a piano piece, why not cut out the middle man and try this first.
Pierre Schwob's massive collection of classical music MIDIs and MP3s. I suspect that most people don't realize how useful MIDIs can be for a quick reference - lacking a recording at hand, this is a quick way to hear what something sounds like. Unfortunately, it looks
like he's put a pay wall up on all the content now (for $6/month), which is a shame, but for certain projects it could still be useful.
Archivmusic is primarily a mail-order CD and DVD company that specializes in Classical music. Why would you use these guys over Amazon, you might ask? Well, they've
got the best-organized catalog on the web. This is the first place to go if you want to find out if a certain piece is available on CD. AND, they offer a lot of out-of-print recordings as part of their "ArchivCD" series.