Nothing too terribly exciting, but the Source Map feature, allowing easier debugging inside minified code seems very useful. Chrome only, but still an extremely slick feature:
http://www.html5rocks.com/en/tutorials/developertools/source...
http://www.html5rocks.com/en/tutorials/developertools/source...
Only for now, fortunately! The co-authors are from Google and Mozilla[1], and work's being done to get support in Firefox as well.
[1] https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiO...