As my 30GB iBook hard drive was getting full, I had to clean out a lot of DarwinPorts apps. However, I discovered that you can install DarwinPorts to a FireWire drive, instead of the internal hard drive. When you install a program, the full package will not install, but just use symlinks instead.
Assuming you have a FireWire drive named FireWireDrive, open a Terminal and type:
$ cd /
$ mkdir /Volumes/FireWireDrive/opt
$ ln -s /Volumes/FireWireDrive/opt opt/
Then, install the DarwinPorts package installer. When you want to install other apps, simply type:
$ /opt/local/bin/port install application_name
The install will then place the app on the FireWire drive, not the internal drive. You'll not notice any difference in speed, etc. Now I have 100GB of space for X11 apps!
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005100718490565