So you do what everyone says you should do: don’t tell anyone your passwords, don’t write them down anywhere… and then you die. Your grieving relatives can’t get access to your online address book, so can’t notify your friends. Or your colleagues can’t get access to the vital work you were doing the day before you were hit by the bus.
Personally, my memory is really bad so I do keep records of all my passwords, whatever anyone says—but certainly not in plain text. They are safely encrypted, so there’s only one master password that then provides access to all the others. There are a number of applications out there that you can use for this: just search on “password storage” or “password manager”. I use SplashID, because it works on my pda as well as my laptop. Which reminds me… I should write down that password in a safe place, where my family would find it in an emergency.