CP/M - predecessor to MsDos, and a de facto standard OS on 8080/8085 (and later, z80) machines with S100 bus. Supported on almost all 8080 and Z80-based machines, including the Dec Rainbow (which also ran DOS), the Commodore 128, and the Amstrad PCW.
MsDos - A reverse-engineered version of/rewrite of CP/M with some added features (and in its first versions, some missing features), MsDos was the text-based OS that shipped with the original IBM/PC.
DesqView - a visual "shell" that enabled the user to do multi-tasking on DOS way before Windows existed.
LotusAgenda - A weird concept invented by Mitch Kapor who was the CEO of Lotus; Lotus Agenda was not quite a database, not quite a scheduler. In fact, no-one really knew what it was but some people loved it.
NortonCommander - A Dos shell that was very popular during the DOS days; it got translated in God knows how many languages and it launched Peter Norton's career. There are clones and ports for virtually every operating systems.
PfsFile - one of an excellent series of different software edited by Software publishers.
RapidFile: It was a flat-file database (coupled with a word-processor) written in Forth and quick as lightning since it operated in RAM mostly. Few people knew it but those who did loved it!
ViEditor - The terse philosophical counterpoint to emacs, vi offers powerful, maybe even elegant, editing capability in a small package.
VisiCalc - The spreadsheet that launched a thousand MBAs, and the first KillerApp for PCs. It started on Apple2.
WordStar - Remember these old commands? Ctrl p; Ctrl K; Ctrl Z. In an earlier (pre-GUI, pre-Microsoft office apps dominance) age, the de facto standard word processor under MsDos, CP/M, and several other operating systems.
WordPerfect - A hugely popular word processor that was emulating the concept of the blank page.
Xtree - It made it possible to locate dos files easily and popularized the (now very common) user interface of a collapsible outline on the left of screen and a horizontally divided right screen section. The upper portion of this showed a file list and the lower portion showed the contents or details of that file. Think how many user interfaces have been spawned by that product.
HuntTheWumpus - A strange game many remember fondly. In this precursor to Infocom-like text adventures, your goal is to hunt down and shoot the wumpus in a network of interconnected rooms. The catch is that if you bumble into the room that contains the wumpus, he will eat you. To kill the wumpus, you have to shoot into his room from an adjacent room; when you are 'close' to his room, you receive the message that you can smell him nearby.
InteractiveFiction - Colossal Cave AdventureGame, ZorkGame, and their successors pioneered the story-driven long form game. A lively amateur community still thrives today, producing amateur-written games that equal or exceed the old classics from the 1970s and 1980s in quality.
StarRaiders - The first three-dimensional first-person space game. For the Atari 8-bit PCs, ran in 8K of RAM; a much less-interesting, lobotomized version was also available for the ubiquitous Atari 2600 game console. Crude by modern standards, but revolutionary in 1980 (about when it came out). The predecessor to Wing Commander and similar games. (If you lost and lost badly, you would be posthumously demoted to "garbage scow captain" or some similar gag rank.)
TetrisGame - One of the most successful games of all times. And still around today!