I think the most common is you have a single password, but you
hash it with the name of the service you're logging into.
For example, if you're password is "puppy" and you're signing-up for HN, your password would be:
pHuApCpKy
And, if you wanted to make it stronger, salt it with some special characters.
p~Hu!Ap@Cp#Ky$
... which is just the shift-characters on a number row in order.
This way, you only have to remember one password, and it is service specific, and pretty strong. No password manager needed.
Of course, I don't do this. I use a 1Password and KeepassX.
