Nifty - Manage projects, work, and communications in one place.
draw.io - Online diagramming application
ZenHub - ZenHub is how the world's best software teams work together. Trusted by NASA, Docker, and Rackspace. Start free today!
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.
Foreword - High impact workshops to help you thrive at startups
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.