Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
draw.io - Online diagramming application
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.