Apart from nostalgia, there is a serious side in the educational value of looking at the older systems on which our modern digital infrastructure is partially based and preventing information being lost forever through data obsolescence and physical decay. Charles Babbage's Analytical Engine, designed in 1837, outstripped the production capacities of its day. Had it been built, this mechanical wonder would have been the world's first general purpose computer. The Fourmilab site contains a wealth of information on the Analytical Engine, including original historical documents by such notables as Ada Lovelace, arguably the world's first programmer. The site features a Java-based emulator with some sample programs.
Source code is available and is in the public domain.
The DOS emulator is still available as well versions for Java and Windows. A programmer's reference is also included as well as some java source code. The first computer sold on a retail basis, it was the definitive minicomputer and was also cheaper than many of its rivals. This site, offered by Bernhard Baehr in Germany, provides a PDP-8 emulator for the Macintosh, released under the GNU public license. A computer built with hobbyist sensibilities but mass consumer appeal, it offered sound and color and could handle up to 48Kb of memory.