[mark_l_watson]

I almost never use Prolog anymore, but I think it is a language really worth learning. Lisp languages, Prolog, constraint satisfaction languages, Smalltalk, etc. may not be mainstream but they will expand your ways of thinking about solutions to problems.

[MrEldritch]

Exactly. In fact, I'd go so far as to say the less people use a language, the less relevant to modern programming paradigms - the more it is worth learning.

The way to expand your sense of what's possible is by learning completely different ways to do things and think about things, and - at least in computing - the best way to find truly alien visions of what computers might become is by looking for the ones that had to evolve completely independently of modern programming paradigms because those modern paradigms hadn't been invented.