This is my attempt to link to a good collection of music available on the web, for free.
Smashing Pumpkins: Machina II
The Smashing Pumpkins' album "Machina 2: The friends and enemies of modern music" was released with a limited run of 25 vinyl-only copies in 2000. Many copies were distributed with instructions to the owner, stating that the album should be ripped and distributed for free on the Internet.
The "album" is really a double-sided 12" and three EPs of b-sides and rarities.
There are several copies of this floating around, sourced from different rips of the vinyl. (Read about the album and the most prominent sources on wikipedia). Several are up on archive.org:
Particular highlights for me on this album are "Glass" (LP version); "Let Me Give The World To You"; "White Spyder"; "Atom Bomb".
Nine Inch Nails: The Slip
Nine Inch Nails released this album under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 in 2008. Originally, the track "Discipline" was written as a single to support their upcoming 2008 US tour. However, Reznor was bitten by the muse and ended up squeezing out an entire album, which he decided to give away.
In my opinion this is actually the best Nine Inch Nails album. In particular, the tracks "Head Down" and "Demon Seed" from an Industrial Rock point of view, but "Corona Radiata" and "The Four of us are Dying" are excellent ambient/electronica pieces.
Two of the tracks from "The Slip", above, but also
Nine Inch Nails: Ghosts I
Nine Inch Nine again! NIN released a 4 EP set in March 2008. The albums is released under "Creative Commons Attribution Non-Commercial Share Alike". The first EP was distributed free-as-in-beer by NIN themselves using bittorrent. You can buy the rest as a digital download from NIN for very little money, or go and find a 3rd party torrent (the whole thing is also on archive.org).
There are also two extra tracks which were not featured on the digital releases but available in "multitrack" source form on the data DVD that accompanied the physical release. "Ghosts 38" in particular is interesting because it's evidently the beginnings of "Demon Seed" from "The Slip".
Soulseek records was an off-shoot of the "Soulseek" peer-to-peer music software's community.
An artist I originally became aware of via the Soulseek records releases. My favourite track is "Perfected Everything", part of his album of the same name but also "Soulseek compilation 1".
Currently KiloWatts is giving away two of his EPs for free for a limited time. "Teknopera" is a three part breakbeat masterpiece.
The Conet Project -- Recordings of Shortwave Numbers Stations. Really creepy. begging to be mixed into Industrial tracks, so Hexane did exactly that (Coded transmission one, two, three). I really like "annulled" and "intimacy", neither of which are sourced from the Conet project.