Scribbles is perhaps the simplest drawing program I've ever tried. The app, created by atebits, gives you 4 tools: brush, move, pan, and zoom. There are 12 brush styles, a color picker, a big slider for brush size, and a layer manager.
It looks deceptively simple on the surface, but when you explore a bit, you'll find some really interesting functionalities that even the advanced Mac freehand drawing tools like Sketchbook and ArtRage don't do well. One is the tracing paper mode, which gives you a transparent canvas so you can position the window over anything on screen. The other is the infinite canvas, which lets you zoom out infinitely to widen canvas by just dragging a magnifying glass cursor. It's pretty wild, and you kind of get a sense of scale of your zoomed canvas when grids are on.
Seems like kind of a fun toy, but I'm sure you could do a lot more with this if you got creative.