This free, open-source extension generates a strong password based on a master password and an input, usually a URL.
So in essence, this is a free, open-source, offline password manager.
This means you'll have a strong, unique, generated password for every occasion, yet only need to remember one password. The master password still needs to be a strong password, but that is your own responsibility. Best of all, it's entirely offline, so your master password never gets sent over the wire.
If you wish to view the source code, you may do so here: