This hint is not specific to OS X, but I think it's worth sharing as it might save you a lot of time and heartache!
If you have a dying hard disk (it might have started making clunking noises, or is reported by Disk Utility as 'failing'), try removing it from your Mac and placing it in an external HD enclosure if you can. You will then be able to change to orientation of the drive (stand it on it's end, or lay it upside-down). If you're lucky, a different position will allow the drive platter to spin smoothly for long enough to get your data off.
It worked for me! In the space of the last hour I've gone from deep depression to skip-round-the-room-singing joy as I've just recovered all of my data from my PowerBook's dying HD.
Earlier this evening the PB's internal drive began to make terrible clunking noises. OS X was intermittently locking up as it tried to write data to the drive. Things were not looking good--my most recent back-up (a bootable clone on an external Firewire drive) was over three weeks old (oops!); I was facing the loss of a fair amount of work. I tried booting from the backup so that I had a stable system and then spent the next couple of hours desperately trying to copy files off the internal drive. I didn't have much luck and the drive was sounding worse.
I guessed that the drive was failing due to the disk platter bearing wearing out (hence the noise) so I decided to swap the internal failing HD with the drive in the Firewire enclosure. I thought that once the bad drive was in the external enclosure I would be able to change its orientation, maybe allowing the drive to spin steadily.
A few minutes later, I rebooted the PowerBook, plugged in the Firewire enclosure with the failing drive and crossed my fingers. At first the drive was not recognized at all. It was spinning, and clunking a bit, but no data was being read. I slowly picked up the enclosure and rotated it. Suddenly, as the drive came into an inverted state, the bearing noise stopped and the drive icon popped onto my Desktop. An hour later and I have all my data safely on another drive!
It's worth noting that I was lucky, but I also had a few factors in my favor:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061027115456243