I can't recall if this has been posted before or not, but check out <http://www.mozart-oz.org>. Oz is the language; Mozart is its EMACS-and-Tk-based development environment.|
Oz is, at core, a concurrent constraint programming language. Oz' developers, however, have reified many other popular paradigms into concurrent constraint programming: OO, logic, functional, and imperative. This might make it sound as if Oz is guaranteed to be a total hodge-podge of a language, but it's not.
Oz also makes concurrency and distribution all but trivial. If Jim Ingham ever finishes Tk for MacOS X, I'll eagerly install Mozart and Oz on my system and dig in.