Most interesting non-mainstream language? [closed]

Question

I'm interested in compilers, interpreters and languages.

What is the most interesting, but forgotten or unknown, language you know about? And more importantly, why?

I'm interested both in compiled, interpreted and VM languages, but not esoteric languages like Whitespace or BF.
Open source would be a plus, of course, since I plan to study and hopefully learn from it.

Answer 1

FORTH was a language designed for low level code on early CPU's. Its most notable feature was RPN stack based math operations. The same type of math used on early HP calculators. For example 1+2+3+4= would be written as 1, 2, 3, 4, + , +, +

Answer 2

PROLOG is a rule-based language with back-track functionality. You can produce very human-readable (as in prosa) code.

Answer 3

How about go? It's brand new, so it's unknown and not mainstream (yet).

It's interesting because the syntax looks like what happens after you put C and pascal into a jar and make 'em fight.

Answer 4

Harbour for dynamic type. Great opition to business apps.

Answer 5

Smalltalk (see discussion linked here). Sort of the grand-daddy of the dynamic languages (with the possible exception of Lisp and SNOBOL). Very nice to work with and sadly trampled by Java and now the newer languages like Python and Ruby.

Answer 6

Boo targets the .NET framework and is open source. Inspired by Python.