Magickly aims to make image manipulation possible as a simple web service. The Magickly API is very easy to use, and completely open source. It requires no API key, and allows a developer to use a lot of different image manipulations on any web-accessible image. Magickly is an open source, simplified version of the Aviary Effects API, which has a contest ending next week.
Magickly is, at its heart, a web front-end for ImageMagick. It supports many image operations, such as rotations, flipping, converting color to greyscale, resizing, cropping, and many more through a simple API. A programmer can either use the hosted version at magickly.heroku.com/, or host their own version using the source. Rather than returning results in a request/response sort of way, all one does to use it is access a URL including the URL of the source image and the operations to be performed. The result is an image file, readable by anything that can read image files, transformed with the specified operations. It is also available as a Ruby Gem.
For such an amazingly useful API, it sure hasn't received much coverage. Finding even a mention of it anywhere is pretty difficult. I hope people read about and start using this one more. It could allow all kinds of interesting things, such as an effective image manipulation program for iPhone or Android using it.