MacPhun makes a boatload of photo editing apps for iOS and the Mac; for the next two days, their meat & potatoes Mac app, FX Photo Studio, is half off at $10.
FX Photo Studio has a clean, easy-to-use interface, a button that allows you to instantly compare your modified image with the original and simple masking tools, and you’ve already got a much better tool than OS X’s own Preview app.
Add over 100 ready-to-go filters (brush strokes, alien skin, 3D-effect, etc.) and the ability to instantly upload creations to not just Facebook and Twitter but also Tumblr and Flickr, and $10 seems like a pretty good deal. Perfect for the casual photographer who wants quick-hit fixes with minimal time and effort.