Nifty - Manage projects, work, and communications in one place.
draw.io - Online diagramming application
Intercom on Product Management - The ultimate resource for aspiring product managers
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.
productboard - Beautiful and powerful product management.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.