NODEPAD by Joe Tannenbaum (source on GitHub)

Z: Make a node

HOVER+X: Delete a node

HOVER+Z: Start edge, Z again to place edge

Hit X to cancel your edge

C: Clear nodepad

D: Directed edges on/off

1-7: to cycle through colors

HOVER + [1-7] to change a node's color

CTRL +/- to zoom in/out