I am the developer of several major addons for World of Warcraft, one of the most famous and influential MMORPGs of all time. My software is currently distributed on CurseForge, with over 250 million downloads.
Addons are third-party plugins that can enhance and change how a game looks or behaves. In the case of this game, this form os software is officialy supported by the game developer, which provides an official API to do so, but is restricted to modifications of the user interface.
This has been mostly a single man job, with the exception of occasional cooperations with translators and other developers for purposes such as localization, software compatibility or contribution to common libraries. Most of the source code is publicly available, as I develop the mods under an open source model. I have also created many development tools for my own use, some of which are freely available on GitHub.
Making addons was my very first step into programming and computer science, when I was 13 years old. At the time, the great majority of addons were not very complex and the market far from profitable. For instance, my addon Scrap was the first junk seller to have an actual interface (others were configured using the command line) and the first addon to have in-game tutorials. The last latter became a common practice of the game developer, which now provides in-game tutorials for new interface elements it incorporates.
Not all the addons I have created are still being maintained. Some became unecessary, while others had to be dropped to make time for other more popular projects. Two examples I want to highlight are Bongos Cornucopia and Cornucopia. The latter is one of the addons I am most proud of, although it never had much commercial success.