"The Killer Date Class!" Version 5.00 (open source code from 1993!)
This is a C++ Date class and is a very early example of "open-source" code being developed by multiple coders around the world who never met. The earliest version of the source code, with the request for public comment and assistance, was first released of 19-Jun-1991, with Version 3.1 released on 3-Aug-1991, beating the first public announcement of Linux by three weeks.