This is a program I've been coding for university. It is a library for the HP50g. This program draws the root locus of a given transfer function and lets you add a regulator, as well as change the proportional value of the regulator and see the new position for the poles of the system for that value.
I've sent it to www.hpcalc.org, too. There are a lot of great programs there. Anyway, the download link is here:
There are instructions on how to install and use it, they should be clear enough for anybody. The source code is included (HP-GCC 2.0 and HP files for packaging the library).