Gain a lot of screen real estate!
For productivity, one of the best but often overlooked feature of Mac OS X is the possibility of hiding the Dock. The Dock is a quick way of loading application or seeing what programs are running but it does takes up a lot of screen space. I used to reduce the size to the minimum but even then, the icon were microscopic and it still took an appreciable amount of space. Dock hiding can be turned on by choosing "Automatically hide and show the Dock" in the "Dock" section of "System preferences". You then only see the Dock on demand when you bring your mouse where it used to be but to the lowest point possible (if it's at the bottom). One of the only downside of this is that it sometimes gets in the way when you resize applications.