I have a question concerning the admirable Stacks Project.

Which comparable projects are there:

approach-wise: "an open source textbook on algebraic stacks and the algebraic geometry that is needed to define them", i.e. projects as "an open source textbook on X and the Y that is needed to define (and understand) X"

technical-wise_{[Side question: which technical framework(s) is the Stacks Project based on?]}

tag-system-wise(using the same tag system as the Stacks Project)

tag-wise(with a significant number of tags shared with the Stacks Project)?

The question concerns also the possible interoperability of several such projects.