The most simple password scheme, and the worst, is to have one password for every site. The best scheme, and hardest to manage, is unique, non-intelligible passwords that you might not even remember. Oplop splits the difference — you name a site, use a common password, and it encrypts new passwords you can then save securely.
Oplop uses the name of the site to "seed" an MD5 encryption of a single master password, unique to the site you're creating a password for. The app offers a thorough explanation of how it works, and even if you can find weaknesses in the scheme, the resulting passwords are almost always more secure and random than what your brain would roll out with. It wipes out all your text when you click away from the page, and there's another precaution built in: a three-colour bar that validates that you have indeed entered the same master password each time into the web app.