To add to what everyone else is saying, most Lisps support redefining functions in a running application. Instead of the code/compile/test loop most languages use, you can create Lisp programs iteratively from inside the program.

It's really quite neat. Emacs is an easy way to play around with it, but if you're a CS language nerd Racket might be a good fit for you. And if you want this type of environment but without all the parentheses and polish notation, check out the Smalltalk implementation Squeak.