This is a list of iOS image filtering, processing, OCR, utilities and helper libraries and frameworks that I have come across for iPhone and iPad development and thought that they may be useful in one places:
1. GPU Image
By Brad Larson, GPUImage is a fantastic fast processing library to apply gpu accelerated filters to both live camera feed, movies or stills. It works on iOS 4 and above. It includes a range of filters such as Sepia, Grayscale, Blur but you can write your own easily.
2. Core Image
This is an apple framework available from iOS 5. It leverages the GPU and you don’t need to know details of OpenGL, OpenGL ES.
Documentation: Apple Core Image Docs