A page that generates different kinds of waves for you to listen to. Defaults to 440 Hz (middle A). Has other kinds of tone generators on the side, too.
ODO has made available several hundred megabytes of Commodore-64 samples, stright from the SID chip.
An online noise and sound generator. Has multiple categories and dozens of different sounds, from thunder to to rain to singing bowls, the wind blowing through a canyon, surf sounds, wind sounds in different locales, different voices, and different brainwaves. You can stack up to five of them in a sequence. You can download them, too! Updated periodically, so check back once in a while. The iOS and Android apps are out, too.
This project emulates the sound of my old faithful IBM Model-M space saver bucklespring keyboard while typing, mainly for the purpose of annoying the hell out of coworkers. Written in C, has a bunch of .wav files. Definitely works on Linux, might work on OSX.