Scheme / Guile / Racket
I've often said that I would love to take @cwebber's brain and shake it to see what all pops out because I know that anything in there will be amazing. One of the languages that he introduced me to was Scheme. Scheme is a functional language that borrows heavily from Lisp. It's a teaching language but still retains a lot of Lisp's power. I haven't explored it fully but I want to get back there again.