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.
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.