[magicofpi]

Here's another possibility: Sketch [1] (by the makers of Fontcase), which seems simple enough also. Incidentally, they also have an iPad app, called SketchPad [2].

For open source, there's an app I've tried a few times called Drawberry [3], which looks something like Pixelmator, but is for vector drawing.

[1] http://www.bohemiancoding.com/sketch/overview/ [2] http://www.bohemiancoding.com/sketchpad [3] http://raphaelbost.free.fr/DrawBerry.html