As of Chrome 6 you can go to a script, double click a location in the script, and make changes. When you are done making the changes they are live in the page.
For all those times we wanted to tweak something and see what it did. Or, add
console.log() somewhere to see what's happening. Or, all those other changes we want to make without having to go refresh the page. We can try them in Chrome and see them live immediately.