People say you never forget your first love. That is certainly true for me.
My longest-lasting and biggest love is Unix. But my first love was the IBM mainframe. I began my computing career on an IBM 370-158 MP mainframe back in 1981, as a junior systems programmer. The 370-158 MP was then a giant computer with two CPUs and 4 Mbytes of RAM, together with a dozen 3350 disks, each the size of a big refrigerator. The CPU and disks alone occupied a big, raised-floor data center and weighed several tons. We used 3270-style terminals to connect through an SNA (IBM's System Networking Architecture, which is not compatible with the TCP/IP protocol).
Booting, or IPLing (Initial Program Loading) the mainframe was a serious issue back then. The machine took about one hour to load the MVS/370 operating system, but that was hardly an issue because the computer was only taken down once in a year or so for upgrades and for building maintenance. MVS/370 was big, complex, ugly, not interactive, and the documentation for it took hundreds of volumes. But boy was it stable, efficient, and fast.
On the above-mentioned machine we had about 2000 user terminals and printers attached over Israel, doing online transactions. Just imagine 2000 users doing serious work on a machine about as resourceful as today's 3COM Pilots. Compare that to the 128 Mbytes of RAM and the minimum of a Pentium II 300-MHz it takes to boot Win 2000 Professional, which wouldn't handle more than one concurrent user, MVS/370 was the master of packed functionality and efficiency. Even Linux is a resource hog compared to it.
MVS/370 was then roughly a 16-bit OS, in 1984 it become a hybrid 16/31 bit OS, meaning that the memory manager could address a 31 bit for a max of 2-Gbyte virtual memory per address space. Then around 1989, the next version, MVS/ESA was introduced being able to address huge amounts of data both on disk and in memory. Lately, MVS/ESA was renamed into OS/390 (MVS was initially named OS/360 back in 1961; a case of the more it changes the more it's the same, I guess).
Although I have worked on Unix ever since then, I always kept an eye on developments in the mainframe world. I was glad to see that OS/390 learned to talk TCP/IP, got a standard C library, and generally more and more metamorphosed into a giant Unix-style OS.
Many corporate data centers still have some old, reliable, and irreplaceable mainframe-based application that can't be easily ported to another OS. For these shops, it is certainly a lot cheaper to just run the emulator and load the applications under it.
I always longed to put my fingers on a mainframe again, but the opportunity never arose. Knowing the open source community (where most of the sys admins of my age, previously worked as MVS system programmers) I just knew that sooner or later an emulator would come out. So, I waited patiently.
Suddenly, about a month ago, google.com reported a slew of sites for the keywords "Linux MVS emulator S/370." Bingo!