Software
The original PDP-10 operating system was simply called "Monitor", but was later renamed TOPS-10. Eventually the PDP-10 system itself was renamed the DECsystem-10. Early versions of Monitor and TOPS-10 formed the basis of Stanford's WAITS operating system and the Compuserve time-sharing system.
Over time, some PDP-10 operators began running operating systems assembled from major components developed outside DEC. For example, the main Scheduler might come from one university, the Disk Service from another, and so on. The commercial timesharing services such as CompuServe, On-Line Systems (OLS), and Rapidata maintained sophisticated inhouse systems programming groups so that they could modify the operating system as needed for their own businesses without being dependent on DEC or others. There were also strong user communities such as DECUS through which users could share software that they had developed.
BBN developed their own alternative operating system, TENEX, which fairly quickly became the de facto standard in the research community. DEC later ported Tenex to the KL10, enhanced it considerably, and named it TOPS-20, forming the DECSYSTEM-20 line. MIT also had developed their own influential system, the Incompatible Timesharing System (named in parody of the Compatible Time-Sharing System, developed at MIT for a modified IBM 7094).
Tymshare developed TYMCOM-X, derived from TOPS-10 but using a page-based file system like TOPS-20.
Read more about this topic: PDP-10