It is often helpful to rotate your display into portrait mode. Tasks like DTP, coding or even reading the web and browsing photos really benefit from the extra screen height.
Display rotation consists of two parts: 1) Physically rotating the display. There are quite a few displays that support this. 2) Rotating what you see on the screen. Mac OS X does this quite well.
The problem for me was that I always needed to open System Preferences manually while turning the second display attached to my MacBook. My monitor does not trigger System Preferences automatically, and I don't know of any displays that support this with Mac OS X.
As described in Graph Paper, it is possible to automate display rotation via AppleScript. The scripts are here. I added a trigger to QuickSilver which enables me to call these scripts via Command-Control-R and Command-Control-L. This solves most of the problem for me.
However, alignment of primary and secondary monitor changes during rotation since I use to have displays stacked above each other. Has anybody come up with a way to change this?
[kirkmc adds: There have been other hints about rotating displays, such as this one and this one (and I was the "friend" who fried a Mac mini trying to rotate the display...).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071003040156502