The problem with this is that an attacker needs to get his hands on one password to be able to try and get into a lot of accounts from that user. This is a high security risk and it is advised to create unique passwords (and usernames) for the websites that you are a member of.

One tool that aids in the creation of unique passwords is the Password Hasher extension for Firefox. The Password Hasher creates a unique password, called Hash Word, that is generated from a unique site tag (normally the name of the website) and a master key provided by the user. The master key can be the same password because it is not stored on the website that you are a member of, only the generated hash word is used as the password on that website.

The benefit is that the user needs to remember the master key and not the unique and complicated hash word. Several options are available to define the size and keys of the hash word. The size can be between 6 and 14 chars with optional numbers, upper,lower case and special chars included.

The benefit of using Password Hasher is obvious. The user still needs to remember only one password if he likes but all websites he is a member of store different passwords that are generated using Password Hasher.