I use the Password Assistant in Keychain Access (/Applications/Utilities) to generate passwords for new accounts. The Assistant is pretty handy, but currently not integrated with any browsers, meaning I have to open Keychain Access, create a new Password Item (File > New Password Item), then click on the key to open the Assistant. I might then have to bounce back and forth between my browser and Keychain Access to find a password that complies with the (frequently unexpressed) password rules of the site.
The Assistant would only appear when two password input fields are detected. What to do when a site doesn’t require confirmation? Maybe check the Keychain for existing accounts at the domain and show the key icon only if none is found?