Discussions, articles, presentations, and blogs for the past few years have given the impression that Open Source on the mainframe is something new. That couldn't be further from the truth. When the mainframe was invented in the 1950's the operating system was included at no charge with the hardware, and mainframe customers routinely shared tools and enhancements with each other - typically on paper tape or punch cards. In the 1960's with the advent of the System 360 this practice continued with the addition of tape reels which were now used. This sharing was across the operating systems on the platform - DOS, VM, OS/MFT and OS/MVT. By the mid 1970's into the 1980's it was possible to find 'mod tapes' available from local users groups (I remember the Los Angeles MVS Users Group) as well as from SHARE and Guide along with their international affiliates. 

I remember attending a SHARE in Houston in the late 70's and seeing number of individuals wearing a Blue Beret and boxes of 3400 tape reels with user modifications and utilities for MVS. 

Sometime in the mid-1970's Arnold Casinghino, working at the  Connecticut Bank and Trust Company began, with the support of his management, to curate and distribute what became known as the CBT Tape. This tape included user modifications and utilities for MVS. He distributed thousands of tape reels over the years. Arnold then turned it over to Sam Golob when the bank where he worked was sold.  For ten years, Sam curated the collection and made the "master tape", while NaSPA made the tape copies. Later, the World Wide Web was available, and Sam Knutson created the www.cbttape.org website with Sam Golob curating the contents. Today  the 'tape' consists of almost 1,000 files, plus another 300 plus files in the overflow area. The 'tape' has also become the repository for many of the specialized 'Mod Tapes' such as the JES2 and JES3 tapes, the Guide ISPF tape, and an electronic copy of the SHARE JES2 Song Book.

As MVS evolved into MVS/ESA and later z/OS, the number of system modifications declined while the number of user exits and utilities increased.

And nearly all of the files are (what we would now call) Open Source, so that both the experienced and the novice systems programmer can take advantage of the function and learn from the source code.

Among the files that you should consider, and this is a VERY small subset, are the following:

This just touches the surface of the wealth of Open Source available for z/OS and demonstrates that Open Source is not something new.