Cause of death: Ethernet, twice
The biggest local area networking technology of the 1990s wasn't Ethernet but a standard backed by IBM called token ring that required expensive and thick cabling to make it work. Robert Madge was the owner and his company was notable for its development efforts in this arena, including licensing its technology to Cisco. When token ring was waning the company moved on to making gear for Asynchronous Transfer Mode networks, which were also killed off by ever-faster Ethernet. The company closed its doors in 2003.
Cause of death: Microsoft and corporate hubris
The first Web browsers were pretty crude affairs, running on command lines and not showing graphics. A team of researchers at the National Center for Supercomputing Applications developed a graphical browser in 1993, and many of them moved on to form Netscape Communications. Alas, Netscape got caught up in the Internet bubble and Microsoft monopolistic tactics, although parts of the code can still be found in Mozilla's Firefox. The company was purchased by AOL, and stopped working on browser technologies by the late 1990s.
Cause of death: pre-announced upgrades
Back at the dawn of the PC era, we didn't have laptops yet, but 30-pound "portable" (or more aptly called luggable) PCs. Adam Osborne was one of the first to build such a device, with 64 kilobytes of RAM and a five-inch monochrome screen more in line with the size of today's PDAs. The Osborne 1 sold for $1800 back in 1981 and the big thing about it was all the bundled software that came on floppies of course. Osborne pre-announced a new version of the PC that killed off sales and his company in 1983.
Cause of death: Fujitsu
The first subnotebook PC that ran on regular AA batteries (and often for days at a time) was the quirky Poqet PC, made in 1989. It looks more like a child's toy today but at the time was quite innovative and was a precursor to lightweight netbooks and Windows CE and PDA devices that are now on the market. There is a tribute Website with lots of links to software here.
Cause of death: Research in Motion
The Blackberry smartphone is now one of the biggest selling such devices. Its antecedent is software from Radiomail Corp. that ran on early wireless devices called Mobidems and used a small HP computer that was called the Viking Express (see photo).
The company was one of the first to understand how to push emails to wireless devices. Its innovations were never patented due to the philosophy of its founder, Geoff Goodfellow. Ironically, after Research in Motion, the company behind the Blackberry, went on to become one of the more litigious computer vendors, it had to pay $615 million to obtain the rights for patents for its device.