Use an algorithmic password. Pick some easy to remember keyword, then work some of the letters of the website into the password so each site is unique. For example, your seed could be "horse", and your gmail password would be something like "hgomrasiel". I've been doing this for ten years and haven't forgotten a password yet. :)
I would like to do this, and I thought about using an algorithm that uses the domain name as the seed, however different sites have different password policies, and expiration times which would make this very difficult to manage in practice. I wish all sites support things like OpenID so I can have one central place to sign in with 3-factor authentication.
This comes up sometimes but I've found it to be less of an issue in practice than you might think. Occasionally I have to make an exception for my bank or gmail. If you do have to make a few variations at least it's only two or three passwords you have to remember instead of a different one for every login.
