I often seen screen shots with areas of sensitive information smeared/blurred to make it unintelligible.
How can I do that image-smearing simply and inexpensively as a Mac user?
The words "simply" and "inexpensively" mean not Photoshop, GIMP, or similar high-end graphics manipulation apps. Normally I just use Preview.app, and sometimes Grab.app or OmniGraffle for my screen shots. I'm looking for something MacPaint-simple.