Sketch Extensions - An official collection of Sketch extensions & plugins
draw.io - Online diagramming application
Remake - A way to make web apps with just HTML & CSS
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.
Sketch Repo - Collection of resources for anyone who uses Sketch
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.