OmniDiskSweeper. (free download)

I just used this drop-dead-easy-to-use utility and was able to easily find and remove over 450 MB of cruft from my 3.1 GB Mac OS 9 partition. Now I have 529.1 MB available!

If you're using Mac OS X, try out this utility, you'll probably find all sorts of crazy files that were hidden in Mac OS 9 that took up insane amounts of disk. (IE's cache file was 130 MB! There were 100's of MB of useless stuff in the "Temporary Items" folder!)