The Living Thing / Notebooks :

(Audio) sample libraries

A particular type of free content, or a particular subclass of data sets, for making your own music.

Audio, General

Impulse Response libraries

Libraries of reverbs, that may be convolved with a signal to make it sound like it was recorded somewhere else - e.g. record a sound in a church then play back other sounds as if they were played in the church. Aside: there is an interesting deconvolution problem in making these things, and possibly an even more interesting one in synthesising them.

Sources, via, e.g. sidebrain:

And ultimately anything can be treated as an impulse response; although you might have to do some fancy footwork to make it sound ok - see above re: deconvolution.