Several Scheme implementations as well as some lesser-known small Lisps qualify.
I think writing a new general-purpose language is something every programmer should try at least once, but don't fool yourself; there's probably an existing language that does what you want.
Now, back to work on that language idea I can't get out of my head involving pervasive generic functions....
I'm half-seriously considering implementing it. The basic form I have in mind is something that looks a bit like Clojure and runs on Parrot. I'm not attached to either of those ideas, but this being 2009, I think it should have some support for parallelism and access to libraries out of the gate.
I think writing a new general-purpose language is something every programmer should try at least once, but don't fool yourself; there's probably an existing language that does what you want.
Now, back to work on that language idea I can't get out of my head involving pervasive generic functions....