trivial-captcha
trivial-captcha is a library for creating CAPTCHA images. That is, it creates distorted images of text that, I imagine, a person might read more easily than a program.

trivial-captcha only exports one function, MAKE-CAPTCHA. The images are created as PGM (Portable Greymap) files which can be processed using the netpbm package, for example:

pnmtopng foo.pnm > foo.png

For source code and a few example images, see Trivial Captcha.