The Void Programming Language
"Programming language is not a templum, it's a workshop." (c)...
Void is an open-source, highly extensible programming language, minimalistic and low-level in its base.
It uses LLVM for code generation and can be seen as a thin (well, in some sense) shell around it.
Extensibility of Void is practically limitless (any syntax/semantics),
restricted only by decidability and your money imagination.