Another one I remember was Arcade Game Construction Kit that in hindsight blows me away with its power on the C64. A full dev-kit for platformer and top-view games including sprite editor/animator, level-editor, tile-editor, sound-editor, etc. All windowed.

I spent days in a beanbag chair with a comfy Epyx joystick (http://en.wikipedia.org/wiki/File:Epyx_500XJ.jpg oh my god that form-factor needs to come back) making games.