Linux only: Open and extract files from ZIP, RAR and 7Zip archives you've forgotten the password to, or never found at the download location, with RarCrack, a free Linux command line utility. Using a brute-force algorithm, RarCrack simply gets to work determining the password for compressed archives, which, in the case of most downloaded RAR files, isn't all that tough. You can point RarCrack in the direction of any special characters you know were used in creating the password, but the standard use—rarcrack yourfile.zip—works just fine in most cases. RarCrack is a free download for Linux systems only; Source files are available at the home page, and Ubuntu Unleashed explains how to quickly compile them.
RarCrack [via Ubuntu Unleashed]