draw.io - Online diagramming application
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
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.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Slack - A messaging app for teams who see through the Earth!