draw.io - Online diagramming application
Material Palette - Generate and export your Material Design color palette
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.
Color Hunt - Curated collection of beautiful colors, updated daily
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Coolors.co - The super fast color schemes generator! Create, save and share perfect palettes in seconds!