This will probably only be of interest to a few people -- namely web developers who use BBEdit and elastic layout techniques. I have written a little AppleScript to convert pixels to ems measurements from within BBEdit.
Elastic layout techniques boil down to using ems instead of pixels for measurements in your style sheets, so that when users change the font size, the whole page 'zooms' in and out. A good way to set that up is to set a default font for the page, then convert all pixels measurements to ems. I set default font size like this (other methods are possible):
body {font-size : 75%; ... }
body * { font-size : 1em;}
The percentage in the font-size can be 100%, 87.5%, 75%, to set the default font size of the page to 'medium', 'small' and 'smallest,' respectively. These are the only percentages that work in the 100 to 75% range -- there are probably smaller and larger percentages that work too, but I've never had to use them.
To convert pixels measurement Xpx to em measurement Xem, you multiply them by a factor (which depends on the default font size). The conversion factors are:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051124024622502