If you've got a PowerBook you use with multiple displays as well as on the road, Mac OS X does a good job of remembering the relationships between the displays, but most applications won't automatically reconfigure themselves appropriately in different contexts. DisplayWatcher can be configured to launch a file (most likely an AppleScript) whenever your monitor configuration switches to a recognized arrangement. This script can adjust whatever application settings you want to adapt to the new display arrangement. I use it to show or hide the dock, and switch to an appropriate DragThing workspace. It can also run a script (or open an arbitrary file) whenever you wake up from sleep, which I use to re-establish my SSH tunnels.
What's new in this version:
This is a Universal Binary which will run natively on the new Intel Macs as well as on PowerPC machines. If you are using Mac OS X 10.3, stick with DisplayWatcher version 1.0.2; the only difference in 1.1 is native compatibility with Intel processors.