The term "Killer App" is often used by technology marketing people to describe the next big application for a new technology, such as e-mail or the WWW being the Internet's killer app. When the first personal computers burst onto the scene in the late 1970's, there were no killer apps. In fact, there were few marketing people in this industry either. The field was dominated by hobbyists and techies, and practical applications were few, notwithstanding some teaching tools and games.
The Apple ][ was selling briskly to hobbyists and educators in 1978, but few yet saw the potential of the personal computer beyond this limited audience. Meanwhile, a Harvard Business School student named Dan Bricklin was working on a novel business tool idea and linked up with MIT graduate Bob Frankston to implement his idea on an Apple ][. When it was completed, they called it VisiCalc.
VisiCalc was the Killer App that transformed the Apple ][ (and Apple Computer Inc.) from a niche product to an essential business tool that virtually every company in America had to have. Today the spreadsheet remains ubiquitous in modern business, and virtually all of this can be traced back to this single invention created in an attic in a Boston suburb.
While VisiCalc, and the company created to sell this tool, Software Arts, are mere footnotes in computing history (Software Arts was later bought by Lotus which was eventually eclipsed by Microsoft), you can read about the creation of VisiCalc at Dan Bricklin's website, and even download an early version of VisiCalc for the IBM PC, which was made available to the public for free through an agreement by Lotus which still owns the copyright.
Today, over 25 years later, the personal computer industry is a several hundred billion dollar business. However, the creation of VisiCalc and the business revolution this started, long before the dominance of Microsoft, remains one of the most important events in personal computing history!