Mainframe

BMC Software
Computers, Software, Operating Systems, Mainframe - BMC Software. Systems software company with emphasis in systems management, mainframes, IMS and DB2.
https://www.bmc.com/
Responsive Systems
Computers, Software, Operating Systems, Mainframe - Responsive Systems. DB2 performance tools and consulting for DB2, IMS, CICS and z/OS.
https://www.responsivesystems.com/
Mainframe Cobol Programmer Help
Computers, Software, Operating Systems, Mainframe - mvshelp.com. Help Boards and Interview Q&A provide moderated technical questions and answers in forums divided along subject lines.
https://www.mvshelp.com/
CBT Tape Home Page - MVS Freeware
The CBT tape is a collection of freeware almost all open-source distribution for the IBM mainframe MVS and OS/390 operating system environment. The entire collection of free software can be downloaded.
https://www.cbttape.org/
Cole Software - Home
Cole Software's z/XDC is a comprehensive and flexible product for developing and debugging source level assembler language programs that execute in z/OS (MVS) operating system environments on IBM mainframes. z/XDC offers the ability to debug assembler programs running in a broad range of z/OS environments. Cole Software's new product, c/XDC, is a source level diagnostic tool for C programmers who develop in mainframe environments.
https://www.colesoft.com/
Software Engineering of America: SEA
Computers, Software, Operating Systems, Mainframe - Software Engineering of America: SEA. Specializes in mainframe data center software.
https://www.seasoft.com/
Compuware Corporation Homepage
Computers, Software, Operating Systems, Mainframe - Compuware. A software and consulting company specializing in assisting the application development process, especially in z/OS mainframe environments.
https://www.compuware.com/