fsck, or the unix/linux command "file system check" is the answer to that horrible pit-in-the-stomach feeling you get when you boot up your computer, and by no fault of your own, you get a message indicating your hard drive is buggered.
When I ran windows at home and at work I had experienced about 8 random cosmic ray-related hard drive buggeries. All 8 resulted in complete rebuilds and tearful attempts to recover data.
Since I've switched to linux I've experienced no less than 5 of these buggeries. fsck saved me from all but one (fucking Maxtor!). One of which occurred quite literally four hours before a large project was due.
fsck was there to save my fscking life.
Thank you fsck, and thank you linux.