Draw diagrams with ASCII tools. Drag boxes, make arrows, add text. As art or as mockup diagramming tool, this is really neat.

From the US National Library of Medicine, a huge collection of free, public-domain images from anatomical atlases. A great wealth of material. Wish it were designed better and that the books were all scanned rather than some photographed.

"Examples of the dashed line in use, and the various visual meanings of the dashed line."