Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
draw.io - Online diagramming application
Sketch Toy - A drawing application that lets you save and share replays of your work with friends.
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.
Sketcharama - Draw a given word and let the other players guess your sketch.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.