Flying Meat's FlySketch 1.6.3 is an odd combo at first glance. It's a simple drawing tool that doubles as a screen capture utility.
Its elegant-looking, self-contained interface includes an adjustable window that opens and sits atop other windows on your desktop, while it operates in the background. A slider lets you adjust the view from transparent to opaque to see images and type underneath the canvas.
Several sophisticated palettes let you apply and change stroke and fill colors, align elements, add drop shadows, and draw connectors. You can annotate your picture with a highlight, draw on it, create boxes and circles, and otherwise create a unique image. You can then use the built-in scripts to e-mail, save, or otherwise transport your picture.
Because the app operates in the background, choosing it is awkward; you have to go directly to the dock to bring it to the foreground, use a key command or the Close button to hide or remove it, or physically grab its menu bar to reposition it with the mouse.