VisiCalc was the first spreadsheet program available for personal computers. 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 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.
External link: http://www.danbricklin.com/visicalc.htm