[calvin_]

I know a friend who's employer was throwing out an E4A. OS/400 is also pretty interesting from an architecture perspective too.

[lambda]

Yeah, it might be kind of interesting to play around with IBM i/OS/400.

Oh, and I guess I shouldn't have called it a mainframe operating system; that's z/OS. IBM i is their minicomputer operating system.

[southern_cross]

Hardware-wise, IBM i can scale up from basically a thick pizza box to something relatively monstrous in size. The architecture and OS are still relatively advanced, too, even by today's standards. Those of us who know its history sometimes think of it as "IBM: The Next Generation", since it was based at least in part on the Future Systems project, a replacement computer architecture that never quite came to complete fruition. IBM has never marketed it as a mainframe-class system, though.

[pjmlp]

I used an AS/400 as operator back in 1994, was mostly taking care of doing backups.

Sadly I never got to program into them, but at least .NET (on Windows), Android seem to share some of the ideas.