Specifications for various motherboard, power supply, etc, system components.
dead link: http://www.formfactors.org/
2020 Update: The old site redirects to an intel.com page but that really doesn't have much about various form factors. Oh well.
From Ars Technica, a collection of FAQs and links for case cooling concerns.
dead link: http://faq.arstechnica.com/?s=3
A large collection of specifications for computer cables and connectors. A little out of date, but good for older devices.
dead link: http://hardwarebook.net/
2020 update. Fixed link,
but the new site is kinda broken, like it's missing a style sheet.
And it gives completely different
content for http:// (real but broken) and https://
(a default site template).
An online aircraft museum with a focus on the SR-71 Blackbird, a long range reconnaissance aircraft. Included is, eg, the entire flight manual.
qz thoughts