Apples hidden AppleDisplayScaleFactor provides a means to set the UI display scaling factor, a feature which has been built into Mac OS X since Tiger (I think) but has never been exploited so far (probably due to the difficulties from moving from pixel-oriented graphics to a more generic concept).
Many applications don't support this feature yet, so setting the factor globally results in a big mess in some of the applications. Setting the scale factor for single applications however helps in saving some screen estate.
I'm running two screens, one is a 24 inch display (1920x1200), the other my MacBook Pro's internal 12800x800 pixel screen. Although this is quite some space, I'm always running low on screen estate. I found that setting the Display UI Factor to 0.85 saved quite a bit of space when applied to the following (for me always running) applications: Mail, iTunes, iCal, Skype.
This can be accomplished by using the following code in Terminal:
defaults write /theapp/ AppleDisplayScaleFactor 0.85
To reset back to the default behavior use:
defaults write /theapp/ AppleDisplayScaleFactor 1
Please note that /theapp/ needs to be replaced by one of the following:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110322011218363