I used to have one password.It was the password to my Unix student account and it was in the mid nineties!
Since then, I must have dozens of passwords for work/home computers, websites, files, etc. Having a truly different password each time is almost impossible unless you use some kind of password safe application. Or you could use some kind of clever formula, I do emphasise on the “clever” because if your formula is to generate the same password with a simple variant at the end of it, a hacker who has access to more than one of your password could find out what that formula is quite easily.
Another issue is the username. Most security warnings are related to users having the same password, although it is indeed true, there is also an issue with using the same username everywhere. I would argue it is more important to start with a known username than a known password.
The recent attack against Sony shows that credentials stolen from other companies/websites can be re-used to mount generic brute force attacks. This is echoed in another recent article about the increasing danger of consoles and their online credentials that can sometimes be the same as those used for corporate use, especially with Windows live ID. I would again argue that it isn’t just an issue with consoles as many people when registering to new websites re-use the username they use the most, their work or home username.
There is however the need for a tradeoff between the highest level security of having a random username and password for each of your login, and something you can use without having to think/consult for/every 5 minutes.
I would start with a different password for every login… and to change them from time to time.