I agree - check /tmp and /var for huge temp / log files... I had /var fill up on me once when someone was trying to break in to my home machine - they generated so many log files, that I ran out of room, and it caused me some problems. Since then, I've always made /var a seperate filesystem...

Also, you can try :
du /var > var.du
du /tmp > tmp.du

Now read the var.du and tmp.du files... it will show you where you're using the most space. Look for big numbers, then investigate the directory it's in.
(I think most systems will represent the total size of each directory in kilobytes with du)

Hope this helps.