Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
draw.io - Online diagramming application
Walling - Walling is your visual space to capture ideas and organize projects.
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.
Nuclino - Nuclino works like a collective brain, helping teams bring all their knowledge, docs, and projects together in one place. It's a modern, simple, and blazingly fast way to collaborate.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.