Pbmplus is a toolkit for converting various image formats to and from portable formats, and therefore to and from each other. The idea is, if you want to convert among N image formats, you only need 2*N conversion filters, instead of the N^2 you would need if you wrote each one separately.

This software isn’t necessarily related to image compression, but it is often quite useful when working with images.

http://www.acme.com/software/pbmplus/