gd Graphics Library
GDIT (GD InTerpreter)
giftool
IX (An image transformation library)
jpegsrc (JPEG Library and Conversion Programs)
libtiff (TIFF Software Library)
MPEG:
Nan's Computer Graphics Page
Ray Tracing:
Sced
Source Code from the book Graphics Gems
Stereograms:
The Graphic Utilities' Site & Version FAQ
Transparent GIFs:
Utah Raster Toolkit (Look for "urt-*")
XPM Format and Library