Hashing passwords
If there's one thing that really annoy me, it's when I forget my password at some random website, ask for a password reminder and get my old password sent back to me in clear text in an email. This shows so many levels of ignorance in the people that developed the system, that I immediately feel like deleting my profile and never come back.
I started thinking about this again today when I read Sony's announcement about the PlayStation Network compromise, and got a bit surprised when they also listed passwords as one of the pieces of information that might have been stolen. Surely Sony can't be that unprofessional, storing passwords in clear text!?
I'm not sure we'll ever figure that out, but anyway, if you're ever finding yourself developing a system that needs to store passwords, please continue reading.