Thumbor is pretty impressive. Not only does it take something like thumbnailing, which is always painful, and makes it easy. It has cool image operation out of the box.
It also features a VERY smart detection of important points in the image for better cropping and resizing, using state-of-the-art face and feature detection algorithms
It even sports an east to use API with urls like:
If you're like me and think thumbnailing is a pain, checkout Thumbor.