The Internet Archive Manual Library is a collection of manuals, instructions, walkthroughs and datasheets for a massive spectrum of items. Manuals covering electronic and mechanical products, instructions on mixing or blending items, and instruction sets for software and computer items are all included.
Having the manual for an item can mean the difference between that item being useful (and therefore not immediately junked) and being forgotten, or replaced with similar products. They also give insight into the intentions and goals of the companies making these products, as well as avenues of troubleshooting and repair.
A semi-famous retailer of scientific supplies and instruments.
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. 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 page to open later in a web browser or clone the sourcecode (https://github.com/johnnesky/beepbox) if you want to tinker with it or incorporate it into your own work.