IBM 709.
[link|http://www-1.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP709.html|IBM 709], a vacuum-tube (valve to Peter) machine, with 16K? of 36-bit words. The memory used ferrite cores (hence the term "core dump"). Besides coding Assembler, one could use Fortran. And, of course, the input medium was punch cards.
The above machine was about to be replaced by an [link|http://www-1.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP7090.html|IBM 7090], a transistor machine, and it was my job to make some experimental I/O gear work with the new electrical interface.
Just slightly later, by a couple month or so, I used the [link|http://www.cedmagic.com/history/dec-pdp-1.html|DEC PDP-1] (There had to be a PDP-1! :)). It had 4K (but possibly 8K) of 18-bit words. Paper tape was the primary input medium for your code. It was a "hands on" machine. It was, in fact, physically the very machine on which Spacewar, the first video game, was written and run.
There was no OS for the PDP-1. You boot loaded from paper tape the program you needed.
[edit] Can't leave well enough alone. :)
