VisiCalc was the first spreadsheet computer program, originally released for the Apple II. It is often considered the application that turned the microcomputer from a hobby for computer enthusiasts into a serious business tool. VisiCalc sold over 700,000 copies in six years.
Conceived by Dan Bricklin, refined by Bob Frankston, developed by their company Software Arts, and distributed by Personal Software in 1979 (later named VisiCorp) for the Apple II computer, it propelled the Apple from being a hobbyist's toy to a useful tool for business, two years before the introduction of the IBM PC.
VisiCalc was, in part, inspired by earlier "row and column" spreadsheet programs in widespread use on systems of several national timesharing companies. Notable among these products were Business Planning Language (BPL) from International Timesharing Corporation (ITS) and Foresight, from Foresight Systems. Dan Bricklin writes, "[W]ith the years of experience we had at the time we created VisiCalc, we were familiar with many row/column financial programs. In fact, Bob had worked since the 1960s at Interactive Data Corporation, a major timesharing utility that was used for some of them and I was exposed to some at Harvard Business School in one of the classes." However, these earlier timesharing spreadsheet programs were not completely interactive, nor did they run on personal computers.
According to Bricklin, he was watching a professor at Harvard Business School create a financial model on a blackboard. When the professor found an error or wanted to change a parameter, he had to erase and rewrite a number of sequential entries in the table. Bricklin realized that he could replicate the process on a computer using an "electronic spreadsheet" to view results of underlying formulae. The development of Visicalc took two months of work by Frankston and Bricklin during the winter of 1978–79.

Click on the screenshot to run an in-browser emulation of the program.