Probably needs no explanation, really good platform for prototyping as it's open enough to do things that aren't necessarily games.
Everything you need to create rich interactive applications under one roof. This is the model for what easy-to-use SDKs should be.
It's like an IDE for doing cool graphical stuffs. For you Pythonistas, check out pyprocessing as well.
Just found out about this the other day, node based madness.
Definitive framework for natural interaction (gestures, etc). A bit of a learning curve, but good to know.
So what are some of your favorite rapid prototyping tools?  And if you say UDK, i swear i WILL hunt you down and punch you in the heart...