Webservices

Email Spam Checker
http://www.mail-tester.com/

Git Hosting
Gitlab - https://about.gitlab.com/ (open source)
Phabricator - http://phabricator.org/ (open source)

Group Chat
Mattermost - http://www.mattermost.org/ (open source)
Slack - http://www.json-generator.com/
HipChat - https://www.hipchat.com/
Rocket - https://rocket.chat/features (open source)

Javascript Editor
jsfiddle https://jsfiddle.net/
Codepen - http://codepen.io/
jsbin - https://jsbin.com/ (open source)

JSON Generator
http://www.json-generator.com/

Online IDE
Cloudnine - https://c9.io/ (open source)

HTML/CS/JS Cleanup
http://www.dirtymarkup.com/

SSL Certificate Checker
https://www.ssllabs.com/ssltest

SSL Check Intermediate Certificate Chain
https://www.sslshopper.com/ssl-checker.html

Fake Email Tester (Send emails to here on dev and view them)
https://mailtrap.io/

Terminal Emulator:
https://hyperterm.org/ (open source)

Note Taking Desktop App
Boostnote - https://b00st.io/ (open source)