Microsoft Excel
Back in the 1970's, when personal computers first came out, many people saw them as expensive toys. Since people already had typewriters, they felt that word processing was not really required. Since printing qualities were very poor, and most apps were still very weak and primitive (and because there was no Internet!), people did not see PCs as being worth the cost.
But then came the release of the spreadsheet application, the first one being VisiCalc. These apps allowed people to do accounting far more easily and effectively than before. Suddenly, businesses and households found very strong reasons to buy PCs. Spreadsheet programs became the first killer app.