Rebol
It's built around the notion of code-as-data, like LISP is. It's adherents have a hard time explaining what its unique benefits are over other languages in that family, and usually wind up shrugging and saying something like "Well the guy who designed the AmigaOS came up with it, the standard library is included and microscopic, and once you 'get' it that will be like taking the red Matrix pill and you'll never want to go back."
The trouble with it is partially that it's quirky and hasn't precisely defined which programmers are its market. But also partially because many of the people advocating it just suck at explaining. :)
But Douglas Crockford used to be a fan, drew some inspiration from it with the creation of JSON, and has suggested people look into it as recently as this October:
"Ted Neward did a really good job of moderating the panel on 'Future of Programming Languages'. At the end of the panel, Ted asked the panelists which languages they though people should be learning in order to get new ideas. The list included Io (Bruce Tate), Rebol (Douglas Crockford), Forth and Factor (Alex Payne), Scheme and Assembler (Josh Bloch), and Clojure (Guy Steele)."
Source: sauria.com
I think it's worth looking at for anyone looking to stretch how they think about language design and extensibility. Probably best to avoid using it for anything too important, though....