Dave's Imaginary Sound Space has absolutely tons of audio production PC/Mac stuff etc such as
The Freesound Project."The Freesound Project aims to create a huge collaborative database of audio snippets, samples, recordings, bleeps"
Drumlab is an online drum game thingy - just drag your mouse to hit the kit.
http://composer.extrajetzt.de."The Composer is a Flash music application. You can handle it like a drum computer with a synced-up synthesizer.