SketchPad was the first interactive drawing program, written in 1963 by IvanSutherland as the basis of his tour-de-force thesis. It included visual constraints, character recognition, freehand drawing with smoothing, and animation.
It is now available as a PDF at http://www.cl.cam.ac.uk/TechReports/UCAM-CL-TR-574.pdf
AlanKay insists that all of user interface design for the last 30 years was done in SketchPad first (and then some). He has the videos to prove it. -- He also credits it as the first ObjectOriented system and an important influence on SmalltalkLanguage.