DUNGEON CRAFT / manikus (open source) / PC
Back in the late 1980s-early 1990s, a company called Strategic Simulations Inc. (SSI) had the rights to make PC games using TSR's Dungeons and Dragons license. They used this license to make a number of games known collectively as the "Gold Box" series, which were RPGs with a unique look and structure and which gathered a pretty strong fan following (and even branched out into a couple of Buck Rogers RPGs). Generally speaking, you created a party of customized characters (hewing to standard D&D rules), and explored the game world in first-person "dungeon crawl" style, but the unique twist was that combat took place in an overhead view more reminiscent of a strategy game, with large sprawling battlefields and often featuring huge numbers of characters.
In 1993, SSI released a game called Unlimited Adventures, which was basically a tool for creating Gold Box style RPGs without any programming experience. It was fairly easy to use, robust, and had a decent interface, and soon developed a following. As time wore on and the internet reached into more and more households, a cult following continued to surround the game and actually grow its popularity to some degree, creating all sorts of adventures as well as hacks and modifications to expand the abilities of the original game engine.
Dungeon Craft is a freeware reworking of Unlimited Adventures, keeping the same basic structure but making improvements to the interface as well as adding a number of new features. The project is open source, and thus the core engine is continually being improved, but it is also open-ended to allow installation of selected modules created by other users as well. User-made games can also be made to stand alone without the engine, and a number are available to play.