Along with Piotr Petrus’ Em Calculator for calculating and converting pixels to relative em units, I would also recommend Brian Cray’s PXtoEM.com.
Rather than just outputting one result, PXtoEM.com displays a whole list of em conversions based on the initial body size selected. This makes for an extremely useful reference during the styling of a website.
This won’t replace Piotr’s Em Calculator which allows for more complicated / nested conversions though, and it looks like he’s actually working on a new version.