Citadel BBS for CP/M computers, with Kaypro 2X modifications.
I've taken the ctdl210.zip sources, cleaned them up a bit, and added configuration and code to support the internal 300 baud modem in my Kaypro 2X computer.
You'll need the BDS C compiler 1.4 to build this. It's available on the Internet, and is apparently being distributed with consent of the BDS C author. Do not use BDS C 1.6, as the C API it implements in its standard library has changed (to be more like the modern C we all know and love). If you wanted to compile using BDS C 1.6, you'd need to port some of the code to the new C API.
Changes made to the source are by Jared Boone. I'm unclear how to license my changes. The original source was placed in the public domain. I'd license my changes as Apache + MIT, as seems popular these days for very permissive projects. But I'm not sure I can do that on top of a public domain project?