A bunch of interpreters for various esoteric languages, together with a framework for interpreter development. In fact, also a bunch of esoteric languages from myself.

I mostly lost interest, because a surprising amount of esoteric languages just doesn't do anything interesting. Uhh, so you are using a stack for values. Oh, and your stack manipulation words a bit different from other stack manipulation words. Big deal. Oh, you are just encoding your program pointer in a weird fashion. Also a big deal.

Those which are actually different then just end up batshit insane, and impossible to capture in one bag. Also this set of esoteric languages is plagued by woeful underspecification, or ambiguous specifications, which make an implementation even more impossible.