Pharo is a pure object-oriented programming language and a powerful environment, focused on simplicity and immediate feedback (think IDE and OS rolled into one).

In the discussion about refactoring etc. at the Agile Tuesday someone mentioned Pharo which is supposed to be quite advanced in all things, just as the quote above mentions. Just watching the first 10min of this video was enough to be convinced that this looks really interesting, I am adding it to my "must watch" list :).