Open Ephys is a site that catalogues open source neuroscience hardware - EEGs, data acquisition boards, EMGs, and other bits of equipment that manufacturers historically never manufactured very well.
The Philharmonia Orchestra has released samples from several dozen instuments under a Creative Commons license (CC-BY-SA). Commercial works which use these samples are explicitly approved.
BeepBox is an online chiptune tracker written in flash. You can interactively edit patterns or enter single notes on a piano style keyboard. It makes it very easy to compose music - it manages scales, keys, tempo, and rhythm for you. It also has a waveform editor with which you can define new instruments. Download your work as .wav files. Seems to use Amiga .mod style patterns. You can also download the applet to open later in a web browser or clone the sourcecode (https://bitbucket.org/shaktool/beepbox) if you want to tinker with it or incorporate it into your own work.