The Developer Tools feature built into modern browsers (Chrome/Safari/FF/IE8+/probably others). Extremely useful when debugging JS. I know the Chrome one also allows remote debugging of your web site/app on Android.
Just whatever you do, remove the debugger (and console.log) statements before committing/deploying!!! I've seen this too many times and it's not nice when you end up throwing exceptions in older browsers that have no idea what these statements mean.
