The Avant Garde Project is
"a series of recordings of 20th-century classical-experimental-electroacoustic music digitized from LPs whose music has in most cases never been released on CD, and so is effectively inaccessible to the vast majority of music listeners today."
They release regular 'installments' in their series via bittorrents which can be accessed through Pirate Bay or Mininova. The torrents themselves are announced on the I Hate Music forum.
Earlier installments, of which there are currently 117!) can be found at the AGP Archive.
This is a very useful resource for anyone interested in the early history and development of electro-acoustic music.