Ladies, gentlemen, fellow geeks and retardz, I present CamlScript.org.
CamlScript is an homage to the old Caml Light system, a lightweight sibling of OCaml and thus a relative of F#. It is currently very orthogonal and somewhat limited in use. But still, it is a toy language with some serious chops. And all of it can run inside your favorite browser.
The language itself features a full type inference engine, user defined datatypes, support for proper tail-recursion, as well as pattern-matching. The webpage features a full-fledged text editor (almost a vanilla Ace editor instance) and a retro-style command line interface (a slightly hacked Josh.js instance).
Please check it out.