Science Adventure Series
Steins;Gate is the second and most popular title of the so-called Science Adventure series. This is a series of visual novels, anime, manga and more, all set in the same universe.
There are currently four main titles: Chaos;Head, Steins;Gate Robotics;Notes and Chaos;Child.
These four stories take place in different times and are not directly related to each other, but some characters are mentioned or appear in the other stories. There's also the Committee of 300, which is supposedly the source of all evil that happens in this universe.
A full overview of all SciADV content can be found on the Science Adventure Database.