Here's another classic tale. More than a decade before the Macintosh and Windows PCs, before even the MITS Altair, there was the Alto, the world's first computer with a window-based graphical user interface. Invented at , the Alto had a mouse, ethernet networking, and a what-you-see-is-what-you-get (WYSIWYG) text processor.

But in 1973 the personal-computer market didn't exist, so Xerox didn't really know what to do with the Alto. The company manufactured a few thousand units and distributed them to universities. As legend has it, in 1979 Steve Jobs visited Xerox PARC, saw the Alto, and incorporated into Apple's Lisa and Mac computers. Shortly thereafter Xerox finally realized its mistake and began marketing the Xerox Star, a graphical workstation based on technology developed for the Alto. But it was too little, too late.

Next: Five more blown opportunities, including Napster, CompuServe, and Craigslist.