Thought I'd share this because I find it useful: I don't want to use the same password on all websites but I can't remember a different password for each new service I register. I therefore use an algorithm in my head to construct the password. Say I want to create an account for yahoo.com. I always use the same user name. For the password, I apply my "algorithm" to the service URL. Eg. my algorithm could be "reverse the first five letters of the URL, make the now last char upper case and append '1337!'". So in the Yahoo example, my password would be 'oohaY1337!'. For gmail.com, it would be 'liamG1337!', etc. This way, I get unique passwords for each site, that are at the same time easy to remember.
[It goes without saying that I am using a different algoritm than the above.]