I recently nearly ran out of hard drive space when, unbeknownst to me, a third-party extension began excessive logging to a log file. However, when I tried to delete the file, which was now more than 8GB, OS X would crash, presumably because the file was too large for it to handle. I even tried removing it and "emptying it" from the command line in terminal, using UNIX techniques I read about online -- but that crashed as well, even when I logged in in single-user mode (command line at startup).
It seemed I could not get rid of the file, until a helpful hint from a guru at the Apple Store's Genius Bar came to the rescue. He said that often when files can not be deleted in OS X, they CAN be deleted in OS 9. I simply rebooted under OS 9, and sure enough, the file deleted without a hitch. Hopefully, this simple tip can help others regain some hard drive space, or sanity, when OS X won't fulfill its deleting obligations!
[robg adds: I'm not sure what the answer is if you get into this situation with a new machine that won't boot into OS 9. Although I have such a machine, I'm not really interested in trying to create an 8gb+ file with which to test this hint -- so if anyone has any advice/experience, please share it...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040311123958186