As I’ve been writing about here for a while now, I quite like Prolog as a general-purpose programming language. It’s a lovely, very high-level language, quite comparable to Lisp (with M-Expressions instead of S-Expressions) and my implementation of choice, SWI-Prolog, has a very substantial standard library, is open-source, and very easy to contribute to. However, it is still quite a niche language, with a very small user-base, especially outside of academia.