VisiCalc

With a name that sounds like it's the favorite spreadsheet of the Visigoths, VisiCalc became the "killer app" of the Apple II, at least for the business market, a feat later replicated by Lotus 1-2-3 for the IBM PC. VisiCalc was developed by Dan Bricklin and Bob Frankston for Software Arts, and distributed by Personal Software, a company then largely publishing game software (one of their ads at the time was headlined "Even at 5:12 a.m., it's hard to quit playing Personal Software&trade; strategy games"), but which later devoted itself so much to VisiCalc and related software that it renamed itself VisiCorp.

VisiCalc didn't really invent the concept of a "spreadsheet"; accountants had been creating spreadsheets on paper for ages, and the clunky mainframe era of computing featured a number of attempts to perform the calculations involved in maintaining such sheets electronically. However, VisiCalc was the first personal-computer program to present a user-friendly interface to visually maintain a spreadsheet with automatic recalculation, and it inaugurated a type of software application that is still one of the major things done on computers now. All of that was done with a first version of the software whose entire executable file was smaller in byte size than many of the graphic files that decorate Web pages now.

Starting on the Apple II, VisiCalc was eventually ported to various other platforms including the fledgling IBM PC, though it lost to Lotus 1-2-3 for market share on that platform.

File format
Reportedly, the native file format consisted simply of a series of commands and data elements in the same manner that they would be typed into the program by a user, so that loading a file simply piped it to the input stream of VisiCalc.

VisiCalc could also save files in Data Interchange Format, a file format invented by the creators of VisiCalc for the purpose of cross-program/platform data exchange.

Software

 * VisiCalc executable download of early PC version

Other links

 * Wikipedia article
 * Review of VisiTran, software to generate VisiCalc files from Applesoft programs