At my first (actual) job, I used Zope2, a Python framework that, for that version, was able to hold code and templates in its database. The database was not relational, but instead, a tree-like structure of objects, called ZODB - zope object database. I see both projects are actively maintained.
In a big tech I had to migrate some API clients that were written in Perl, while some template rendering was done in Mason. What stuck with me, was that Mason scripts had some kind of delimitation of execution context; they had some kind of header that was being executed in a faster manner than the rest of the script. Can't remember much.
In spring 2022, I went through Primeagen's Solidity class on frontendmasters when I was doing "my research" on non-fungible tokens and blockchain tech. I don't see myself coming back to that language, but it did showcase a framework in which operations matter in terms of cost. Directly. In 2022 I also watched Richard Feldman's class on Rust - the way you could "cast" the life span (lifetime) of the allocated memory for a variable to an annotation ('a) - that, that stuck with me. Probably, on this list, Rust has the highest potential to find its way into my keystrokes again.
Reminding myself of computational cost and memory brings me back memories of all the obscure development environments I went through in my 4-year Computer Science Bsc - Assembly (mnemonic processor instructions), Matlab (actually Octave, useful for running numerical algorithms that approximate or sample continuous signals), Verilog (simulating simple logical circuits or more advanced electronics, using static gates as well as clock based circuits). Then we have the other programming paradigms that we experimented in, as students: Prolog (solving the Wolf, Sheep And Cabbage Game), Clips and Haskell (functional programming).
Can you top it? :) What's your less-popular development experience?