draw.io - Online diagramming application
UI Movement - The best UI design inspiration, daily
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.
Design Hunt - Curated design inspiration for everyone
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
UI Jar - Hand-picked design inspiration for your real life projects