I wrote the Chrome extension -- it's just a nicer package on a one line hack someone (Pete Hunt? not sure) tweeted. The one liner modified the current DOM nodes while the extension inserts a stylesheet so it will highlight both current and future nodes.

It doesn't highlight at the component level yet. The React dev tools are good at hooking up to components (but I don't know if they rely on dev builds). It would be fun to try to get it to the component level.