This one is epic: A cross platform communications library dating back to 1996. Heck, I even changed my last name since then.
It would compile on Siemens BS2000, Tandem Guardian (sic!), UNIX and Windows NT and implemented a OSI-inspired remote function call protocol on top of TCP/IP.
Herlitz, a major stationary company based in Germany, used the software for many years. Every single delivery item passed through this piece of software on their way from the sales systems (BS2000, later SAP R/3) to the logistics system (Tandem).
The last binaries were eventually removed from the production systems in March 2011. I'm putting it hereby into the public domain.
Enjoy! Martin Rehfeld