We don't have a serious music discussion forum, SO here we are.
To get us kicked off, I know you're all familiar with the blogosphere, but specifically there exists the musical blogosphere, where people upload out of print/rare/old vinyl and cassettes for visitors to download for free.
The best entry point to that world is WFMU's Beware of the Blog, especially Doug Shulkind's Give the Drummer Some's installments which acts more or less as a meta-blog filter. He posts everything from Prohibition era jazz to Sub-Saharan Afro-Funk to old Appalachia fiddle music and EVERYTHING in between. I know I've came across a few gems that he's posted.
Part 1 of 3 of the official Portal 2 soundtrack free for download