The Manchester Small-Scale Experimental Machine was the first computer to store programs in memory and was built at the University of Manchester in 1948 by Williams, Kilburn and Toothill.
The machine had 32 words of 32 bit memory, one register and supported 7 instructions:
|100||JRP||c+s, C||Relative Jump|
|010||LDN||-s, A||Load and Negate|
|011||CMP||Test||Skip if Negative|
With only 7 instructions the SSEM makes an ideal system for an emulation project. A basic simulator can be written in under 40 Intel x86 instructions. If you're tempted to write your own, the SSEM Reference Manual will come in handy. :-)