You run the risk of making it a lot easier to figure out your password for other sites once one of them is known though, right?
Say you create an account at fakecompany.com where they happily and naively save their passwords in plain text in their database. Wouldn't it be easy for that site admin to then figure out your facebook.com, for example, password?
I do this too. You just have to make sure your passwords have sufficient complexity, yet have an algorithm that is easy for you to remember. Just start off a base, let's say oUb$r8!A. Now let's use a simple algorithm of taking the first letter and last letter and injecting it somewhere into the base. So your new password for amazon.com is oUAb$r8!nA. For facebook.com it's oUFb$r8!kA. To the casual observer it looks just like any other password and it beats using the same password over and over again. You can of course make it even more obscure by changing the algorithm, just as long as it's easy to hash out in your head.
Say you create an account at fakecompany.com where they happily and naively save their passwords in plain text in their database. Wouldn't it be easy for that site admin to then figure out your facebook.com, for example, password?