Better Notes - Simple notes app that ties notes together with #hashtags
draw.io - Online diagramming application
Whimsical - The visual workspace for teams.
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.
Great F*cking Startup Advice - Strongly worded suggestions for startup founders
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.