[Cacti]

Never used Clojure but Boo has pretty good support for this, including an interpreter and repl hook.

The compiler and parsing pipeline is exposed, there is an interpreter built-in, you can hook into the assemblies, etc. I have a console in my game that does input/output from a Boo interpreter and it is exposed to everything in the game scope with no real extra effort on my part.

Only took a few hours to set up, most of which was spent either writing a UI for it or sorting out some compatibility issues (Unity doesn't come with a copy of Boo.Lang.Interpreter).

It's pretty sweet.