First place to look is in /var/log. Can be lots of useless crap in there.
Second place, /tmp. (Although most Linux distros delete the contents of /tmp on reboot)
And if you really want to see what's chewing up your hard drive space issue the following command:
$ cd /
$ du -h | more
This will give you a rather lengthy listing of ALL your files on the entire hard drive.
You'll have to hit the <return> key to scroll through the listing.
Hope this helps.