My Unix experience prior to OS X is almost entirely Solaris. The root level of any Solaris partition has a directory named lost+found that you can't delete (or use!) that's "always" empty. I ignored it until recently. My boss' workstation wouldn't boot two months ago, and lost+found was FULL. It turns out, as a gross simplification, that that's where files go if the hard drive is corrupted where that file is stored.
My Pismo running 10.2 has been very cranky lately, crashing a lot. I did some research and decided that the hard drive was corrupt. Before I ordered a replacement, I took a look around my backed-up files. Lo and behold, the root level had a folder named Lost and Found with many files! Most of those files could be associated with recent crashes. That clinched my diagnosis (and order).
I would like to pass on this information that I have learned twice in the past two months: if there's a Lost and Found folder (or a lost+found directory with files in it), go buy a replacement hard drive!
For more information, I learned about lost+found at docs.sun.com's man on fsck and product documentation. May you never need to know this ... and may your search find this hint otherwise.
[robg adds: This is also covered in OS X's fsck man page. I've never seen one on my machines; if I ever do, I'll (a) thank my recent backups for saving my data, (b) try DiskWarrior to see if the drive can be repaired, and (c) probably go buy a new drive!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040722160538864