Okay, so you'll probably still forget them. But with GPass, you can find them again in an instant.
GPass is a quick tool that lets you type up a list of passwords, locked away with a master password. It is easy enough to use: Just click the Add button, and give it a name and a password. If you want, you can set the password to expire so that GPass will tell you when you need to change it again.
And for indecisive folks, or those who like to memorize random characters, it includes a password generator.
The only thing missing, strangely, is a way to print your passwords for a stow-away hard copy. But, for what it does, it does good. Just remember to copy the .gpass folder if you erase your hard drive if you don't want to re-enter them all again.
(You'll probably find this tool listed as "Password Manager" in your distribution's package manager.)