https://travis-ci.org/ Very commonly used tool to test and deploy code

https://www.ssllabs.com/ssltest/analyze.html Ensure HTTPS has been set up correctly.

https://greenkeeper.io/ Automatically update NPM dependencies for Node.js projects

http://stateofjs.com/ Data-driven charts on which choices are popular in the JS ecosystem.

https://developers.facebook.com/tools/debug/ See what Facebook's crawler scrapes from your site and how to change Facebook previews of your site

https://search.google.com/search-console/mobile-friendly Google's test for mobile friendly sites

https://www.google.com/webmasters/tools/home Google Webmaster Tools

http://devdocs.io/ Framework and tool documentation aggregator

https://hunter.io/ Find emails that have been posted publicly on a domain

http://shields.io/ Hipster badges on GitHub READMEs

What other sites do people on HN have bookmarked?