On-Screen Ruler - lightweight, free, semi-transparent ruler
Snapchat - Real-time picture chatting
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS