EDSAC Simulator.

EDSAC (Electronic Delay Storage Automatic Calculator) was one of the very first computers ever created. It ran its first programs on May 6, 1949 calculating table of squares and a list of prime numbers.

Martin Campbell-Kelly of the University of Warwick, England, created a neat software to simulate the EDSAC computer you can download and play with:

The Edsac simulator is a faithful software evocation of the EDSAC computer as it existed in 1949-51. The user interface has all the controls and displays of the original machine, and the system includes a library of original programs, subroutines, and debugging software. The simulator is intended for use in teaching the history of computing; as a tutorial introduction to the classic "von Neumann" computer; or as an historical experience for current computer practitioners.

Link (Thanks Cary Groneveldt!)