VisiCalc
VisiCalc was the first spreadsheet program available for personal computers. Indeed it was the "killer app" that turned the microcomputer from a hobby for computer entusiasts into a business tool.Conceived by Dan Bricklin, refined by Bob Frankston and distributed by Personal Software Inc. in 1979 (later VisiCorp) for the Apple II computer, it propelled the Apple from being a hobbyist's toy to being a much-desired, useful financial tool for business. This likely motivated IBM to enter the PC market which they had been ignoring until then.
Legend has it that Bricklin was watching his university professor at Harvard Business School create a table of calculation results on a blackboard. When the professor found an error, he had to tediously erase and rewrite a number of sequential entries in the table, triggering Bricklin to think that he could replicate the process on a computer, using a blackboard/spreadsheet paradigm to view results of underlying formulas.
Later, more powerful clones of VisiCalc include SuperCalc, Borland Quattro Pro, Lotus 1-2-3 and Microsoft Excel.
Despite being such a revolutionary idea, Bricklin was advised (probably incorrectly in the light of subsequent developments) that he would be unlikely to be granted a patent for it, and so failed to profit significantly from his invention.
VisiCalc can still be downloaded for use on a personal computer - http://www.bricklin.com/history/vcexecutable.htm
