While not directly related to hacking in any fasion, this is just a cool utility, anyone and everyone should make use of.
Par2 makes parity files for various archives, primary used on newsgroups, par/par2 can greatly increase redundancy on compressed archines.
This app has many uses, some not so obvious. Particularly, when I make optical media backups I prefer to compress them, password protect, par2 the file, then burn. This has a few advantages as cd's and dvd's are prone to scratches, the par files can recover a bad rar, while still maintaining your password over the archive. This adds security and redundancy to the backup.