If you’ve ever come across an encrypted/protected pdf (or forgotten a password to one of your own, in my case), it’s a huge pain to try to open the file (impossible, in fact, without the password). Fortunately, some clever people have written an easy-to-use PDF decrypter called GuaPDF (“Guaranteed PDF Decrypter”).
This tool (on Mac, Windows, and Linux) can break 40-, 128-, and even 256-bit keys (although those would probably take a while…). The newest version even supports GPU acceleration through CUDA.
I’m not advocating breaking the security of some files you aren’t supposed to have access to, but this tool can be a lifesaver if you’ve forgotten your own passwords. For example, I was emailed some password-secured tax forms, but never what my password actually was—this tool allowed me to quickly open the files.