Forth
Forth, sometimes also FORTH, is a programming language known for exceptional simplicity and beauty, even when compared to the various dialects of Lisp. It exists since basically forever, or more exactly in 1969. Forth stands for "fourth generation" (of programming languages) and was shortened and capitalized to FORTH due to technical limitations. It is stack-based, untyped, and uses RPN.
The "stack-based" part means that the whole experience programming in Forth is about manipulating a stack, and you do such with a small set of "words", think language elements that you can extend yourself. Depending on the implementation, you get certain "words" to build to new words onto.
Forth is, not unlike old BASIC systems, not only a programming language, but also a whole operating system (in a sense). Moreover, Forth can be worked with interactively, similarly to how you would use Common Lisp/Scheme interactively.