Looks like Yellowdog Linux is discontinued, but at the time it was great: http://www.fixstars.com/en/technologies/linux/

One of the best features of that distribution was none of the standard pre-compiled root-kits would run on it, plus all the buffer-overflow attacks had to be customized for PowerPC, something few script kiddies had the capability of doing. It made for a surprisingly resilient system at the time.