Lisp is neat in that the core language has mechanisms for extension built in. Do you know if any of the other up and coming languages have this same feature or

Lisp is neat in that the core language has mechanisms for extension built in. Do you know if any of the other up and coming languages have this same feature or capability? Can any of the stack-based Forth-like languages? Smalltalk is probably the closest thing to Lisp in this respect, but it too has been overtaken by events.