Sorry for being the obnoxious fanboy of another language, but Lisp + Slime + Emacs has had this feature since forever: By including some swank-updater in your main loop you can have this functionality too. Example: http://www.youtube.com/watch?v=6pMyhrDcMzw

Mathematica is also an inherently interactive programming language, with images, graphs, vector graphics, and GUI elements being first-class values.