FreeRTR is an IP router software package based on the FreeBSD Unix system. It is trimmed down to fit and run on a single floppy based PC (386 or better), and comes with an easy to use configuration utility. There is more information in the README file, and the software can be downloaded by selecting the appropriate package below:
Of course, none of it would have been possible without the existence of FreeBSD, whose contributors are too numerous to mention. It can be obtained at freebsd.cdrom.com.