jgraph/drawio: Source to www.draw.io
Client-side diagram drawing app. Open Source.
Excalidraw
Simple online drawing tool that generates diagrams that appear to be hand-drawn.
Iceberger
How would that iceberg really float? Lots of illustrations that show "the $FOO iceberg" show a berg floating in an orientation that's very unlikely. This site lets you draw the vague outline and see how it might really end up.