One of my proudest possessions is a copy of the original version of VisiCalc, the first spreadsheet program which Dan Bricklin and Bob Frankston wrote for the Apple II (and which was the reason that machine began to sell like hot cakes: people wanted the software and, when told they needed an Apple machine to run it, bought one of those too). If you look at Excel (and, before that, Lotus 1-2-3 and Microsoft’s first effort at a spreadsheet, Multiplan) you’ll see that each was essentially just an attempt to do a better VisiCalc. And of course they were able to do it because VisiCalc wasn’t patented. Dan Bricklin has been musing about this on his Blog.