Whats interesting about this list is how much of what we consider common today was based on innovative thinking of the 1980s and 1990s.
Feel free to nominate your own products and companies in the comments.
Cause of death: corporate hubris and Borland
The venerable dBase II (there never was a dBase 1, it was all about marketing that v.2) was in the early 1980s the most popular database programming language for PCs. Its language was studied around the world and used to build thousands of custom programs for all sorts of applications. Eventually, the company was sold to Borland in 1991. Borland had a competing product called Paradox but by then the original developers had left Ashton-Tate.
Cause of death: better integration of peripherals into PC motherboards
Back when PCs still were 8-bit, they didn't come with much in the way of RAM or peripheral ports. AST was one of the first to get into this business, and produced a wide line of expansion cards. The company's name is from the initials of its three founders, Albert Wong, Safi Qureshey and Thomas Yuen, who in their time were as famous as Brin and Page are today. The cards typically had a whopping 384KB of RAM, serial and parallel ports, and a backup clock battery. But as PC makers stuffed more into their boxes, the compelling business for these cards vanished. The firm moved into the PC OEM business itself, and eventually, they were acquired by Samsung in 1996 and disappeared completely by 2001.
Cause of death: Microsoft and corporate hijinks
When local area networks were in their infancy, Banyan was helping to build large ones with interconnected servers spread across multiple cities. They had a killer directory services component (called StreetTalk) way before Active Directory was even though of at Microsoft, and built their own server hardware for many years until Compaq and others convinced them to become a strictly software company. They also trained Jim Allchin, who moved to Microsoft to work on Windows NT in 1990 (and ironically that product hastened Banyan's demise). The company had a loyal customer base but that wasn't enough to keep them going much past the new millennium.
Cause of death: 3Com
Before there was ever a Cisco, one of the first companies to develop network routers was a company founded in 1981 by Judy Estrin and Bill Carrico called Bridge Communications. The company merged with 3Com in 1987 and lost its lead to Cisco and others in following years. Ironically, Estrin went on to become the CTO of Cisco in the late 1990s and founded a series of several other networking vendors.
Cause of death: corporate hubris and malfeasance
At its peak it was one of the largest Ethernet infrastructure providers, and the company was one of the first to get behind the early 10BaseT Ethernet standards that allowed for a centralized hub rather than the less elegant coaxial cabling of earlier networks. Also notable was the company's founder Craig Benson, who went on to become governor of New Hampshire, one of the first tech leaders to move into the political sector. The company lasted until 2000, at which point it became known as Enterasys, where some of its products and support still resides.
Cause of death: internal and Internet competition
When LANs were still a new thing, the dominant email system that was used on them was cc:Mail. At its height more than 2 million desktops were using the system. Lotus Development acquired the company in 1991 and the final version was released in 2000. They were an odd fit for Lotus and Notes/Domino, and never were integrated into their culture. And as the Internet took on more importance, and Microsoft put resources into Exchange, corporate email systems moved away from cc:Mail to other options.
Cause of death: IBM
The first clone of the IBM PC was the Columbia MPC 1600 in June of 1982. It improved on the original IBM 5150, as it was known in IT circles, with additional memory and expansion slots. While technically the company is still in business, they stopped making PCs in 1987. Ironically, it was IBM's openness initially that got the entire PC entire started, and ultimately led to IBM selling its PC business to Lenovo a few years ago. Columbia was also an innovator of the Small Computer Systems Interface, which it licensed to hard drive maker Western Digital.