netpbm, originally by Jef Poskanzer, is good software. Simple Unix command line tools for images: convert formats, scale, manipulate. Long before Photoshop and Gimp there was pbmplus, and long after there still will be.
I was surprised to learn that after 15 years the netpbm team has added a new format, PAM. It's a superset of PBM, PGM, and PPM: the header now has keywords next to values and a new "tupletype" keyword is added to specify "BLACKANDWHITE", "GRAYSCALE", or "RGB". The purpose of this seems to be to support alpha channels in netpbm. Tuple types "RGB_ALPHA" and "GRAYSCALE_ALPHA" are on their way.