Start your program in your IDE's debugger (or point it to your application server where your program is running) and you can change the code immediately, rerun your methods, change variables, inspect the objects, etc.

It's more than a simple REPL, it's a full-blown debugger and it comes really close to what you have in a SmallTalk image where your code, data, and environment is all the same.

Sound quality isn't great but hopefully it demonstrates what I'm talking about.

I find iPython interesting but haven't figured out how to integrate it into my current workflow of Sublime/Terminal.