Aphex Twin Loops is
a source of techno samples. The samples themselves are here. (8-bit .au files, unfortunately.)
If you're seriously interested in adventurous music,
you probably already know about
who publishes new music CDs and books.
They have a
page full of interesting mp3 excerpts.
Many different processes, from devastating earthquakes to crumpling
pieces of paper to magnetizing magnets to glitchy, noisy music,
make crackling noises.
Here is a pretty good
web site devoted to explaining the phenomenon scientificly and
simulating it on a computer.
From January to March, Humpback Whale season in Hawaii, you can hear them singing, live, thanks to the Jupiter Research Foundation's hydrophone. (Found here thanks to Scot Gresham-Lancaster.)
bremsstrahlung | 2.1 C7 is a record label specializing in lower case sound -- music made from very small sounds. The site design is a lot of fun.