This is the main website for Aerialod Slopefield, a tool which creates virtual landscapes based on mathematical functions. You can then load these in Aerialod in order to generate images like the ones above.
These are two separate tools. Aerialod Slopefield is a command-line tool which only creates the landscapes – but you can't visualise the files without Aerialod, which is a wonderful rendering engine which also provides a graphical user interface. Moreover, Aerialod Slopefield is also released separately, by a different author, and under a different license from Aerialod. Specifically, Aerialod Slopefield (the tool you're looking at now) is a really simple, open source (MIT license) tool written by myself, Bogdan Stăncescu. On the other hand, Aerialod (the renderer needed to actually generate the images) is a free (as in free beer), closed source tool written by @ephtracy.
I honestly believe both tools are intuitive enough for someone who understands the underlying math and technology that you could generate your first rendering 15 minutes from now. Start here: Aerialod Slopefield.