You can use a single password, combined with the url of the website, to generate unique passwords for each website, via a hashing algorithm.
The advantage of this system is:
- only one password to remember
- if a website gets hacked, that password can't be used on other websites, and can't realistically be used to obtain your master password, assuming they even know which algorithm you're using, which is unlikely
- unlike a password safe, you don't need to handle making backups, replicating the backups around, and so on