On the same line, here are few more possible entries:
G’MIC
An open and full-featured framework for image processing. It allows to convert/manipulate/filter/visualize generic image datasets, from 1d scalar signals to 3d+t sequences of multi-spectral volumetric images.
VIPS
A fully demand-driven, threaded image processing library with no image size limits and with good support for color. Compared to similar libraries, libvips runs quickly and uses little memory.
PhotoFlow
Raw and raster image processor featuring non-destructive adjustment layers and 32-bits floating-point accuracy.
Feel free to edit/correct them!