The Hercules System/370, ESA/390, and z/Architecture Emulator
hercules-390.eu
hercules-390.eu
Hercules is pretty neat, and, for a very portable pure emulator, is reasonably quick as well. I definitely recommend giving MVS 3.8j a spin since it's readily (and legally) available.
The platform has a big image problem. I personally blame IBM for not maintaining relationships with their education customers in the early 80s. The high cost of the hardware drove universities to adopt the new and exciting (ie cheaper) distributed platforms, and thus breeding a generation of distributed-minded graduates never even hearing about the mainframe platform except in history books. I know, because I was one of them.
Now the industry is now facing an exodous of their mainframe experts as most near retirment. The average age is something like 55-60, ready to retire [2]. There just isn't any young, talented mainframers being educated. IBM has realized this, and they've started the "IBM Academic Initative" to partner with schools and incorporate mainframe into their curriculum. It's a start, but the damage is done.
The technology, however, is top notch. The closed-source software/hardware model provides a lot of performance gains over the open source model. z/OS is updated every year and IBM releases new, faster hardware every year. And it's not always more expensive. An EC10 can consolidate several racks of Linux blades into its zVM hypervisor (btw IBM has been doing virtualization for 30+ years) while processing 145,000 credit card transactions per second into a single refrigerator-size machine 24x7x365.
If you are looking for a new career, consider mainframe [3]. It's the backbone of financial, retail and data processing industries. It may not be as glamorous, but it's not going anywhere. And when all that old mainframe talent retires, you can write your own check.
[1]: http://en.wikipedia.org/wiki/Jay_Maynard [2]: http://www.businessweek.com/stories/2010-08-03/big-tech-prob... [3]: http://www.forbes.com/sites/rogerkay/2012/01/30/kids-see-a-f...
Taking the IBM courses to fully qualify as a dinosaur herder costs as much as a computer science degree.
The other problem: kids aren't dreaming of maintaining 360 assembler, COBOL or PL/I applications.
[1] http://dtsc.dfw.ibm.com/MVSDS/%27HTTPD2.ENROL.PUBLIC.SHTML%2...
[1]: http://www-01.ibm.com/software/success/cssdb.nsf/CS/JSTS-7JE...
Wait, what? If they changed the license right now to an opensource one, the software and hardware wouldn't slow down at all because of that.
So can a couple of beefier blades running VMware, for a fraction of the cost.