I use Webpack's Hot Module Replacement [0] along with Redux [1] and React. It's extremely useful for iterating on user interfaces. You just save and your styles and markup update without having to reload.

Furthermore, Redux has a devtools library [2] (you can view a gif of it in action in the repo) that lets you do time-travel for debugging purposes. I don't use it much, but it can be occasionally useful.

[0] http://webpack.github.io/docs/hot-module-replacement-with-we...

[1] http://redux.js.org/

[2] https://github.com/gaearon/redux-devtools