turn them into a large external FW hard drive
Large, indeed. I can't think of any other FW hard drive enclosure that's anywhere close to 19" and 30" deep.
one good use...
I contend this isn't a particularly good use... or at least not a very efficient one.
Your typical FW hard drive enclosure draws about 1A at 12VDC, or approximately 1/10th the power of an idle XServe G4. You're spending a lot of power spinning the fans, powering the DIMMs and driving all the other components that are irrelevant when just using it as a hard drive. You'd save space and money taking the drives out and putting them in a regular enclosure. That way you can even turn them off when not needed, without having to go through contortions when you power them back up.
Granted, it doesn't have the 'cool' factor any more, but it depends on what's important to you.
Are there any other good uses that you have discovered
Given their weight I could see them having use in a personal gym. Just got to work out how to affix them to the barbell.