The Keychain Bookmarklet application is a simple tool for storing and auto-filling passwords on webpages. It is more of a proof of concept than anything, and doesn't support encryption (yet). It was originally written for use on the iPhone, since MobileSafari does not include any sort of password management or auto-fill.
The Keychain Bookmarklet Manager generates a bookmarklet that will automatically fill in login and password information when clicked while on a matching website. For example, if add an entry with domain name "google.com", username "example@gmail.com", and password "password" when I navigate to the google.com sign in page and click my saved keychain bookmarklet, the previously entered login and password information will be automatically filled in.
You can edit an existing bookmarklet by loading the manager, then clicking your previously created bookmarklet. Once you are done editing the entries, simply replace the old bookmarklet with the new one.
The manager runs entirely on the client computer, so your passwords are never transmitted to any servers (except of course when you auto-fill). However, encryption of the password database in the resulting bookmarklet is not implemented, so if a generated bookmarklet is somehow stolen, your passwords will be viewable.