Can I Use: Browser support charts for that new hip web technology that you want to use. Includes very basic geographic reports.
Mozilla Developer Network Web Docs: Great for assessing the real world implications of various web elements down to the CSS selector and/or HTML tag level.
HTTP Statuses: Beautiful references for each and every HTTP status. It isn't a reproduction of the specs.
Ruby Toolbox: Meta-tool for the Ruby world.
dply: Free servers with 2h lifespan.
explainshell.com: Parses real shell commands and explains each option
ngrok: Expose a local (dev) server to the Internet. Even behind a firewall.
BlackBox: Store secrets securely in git repos.