Haskell: where the great ideas are coming from, although early implementation decisions have made those a bit painful to use (language extensions). It's probably the most "mind-bending" language out there so I recommend it for growth.
Ocaml: excellent and very simple language, with a kick-ass garbage collector. The worst thing about it is the associated build system, OMake, which is almost as ugly as Make. Most people who are turned off by Ocaml are actually turned off by the build environment or by the Ocaml team's resistance to improvement. (The standard libraries are really terrible; if I recall correctly, List.map isn't even tail-recursive).
Clojure: Great compromise between power of language and library support, because it runs on the Java Virtual Machine. It's a Lisp, so it's dynamically typed. It also has some warts related to its Java roots; for example, 43 and (biginteger 43) count as separate keys in a map. However, these are easy to work around.