download omnidisksweeper from
www.omnigroup.com/applications/
it is free and will help you to find where your data went...
I'm betting that it is either cached webpages OR you have something like poisoned installed and don't realize it...
I would also delete your caches and fix permissions/fsck and that whole nine yards...
then reclaim any lost space with the app I referenced above...
you should also delete the folder ~/Library/Safari/Icons/
it keeps LOADS of files and slows web browsing considerably... see
here