SmartUtils Easy Password is a free and portable tool which can quickly generate passwords for any site based on its URL.
To use the program you must first think of a master password. This must be easy for you to remember, but extremely difficult for anyone else to guess.
After that, though, all you do is enter the master password, the site URL and (optionally) your login, and on clicking "Generate" a secure password for that site will be generated for you.
This approach has several advantages. In particular, it means you only have to remember a single password, and that will be enough to generate logons for all your websites. And Easy Password also doesn't need to store your passwords, so it can be run from a USB stick on any convenient PC.
But on the other hand, if someone spots your master password - they're standing in the background when you enter it, say - that will put them a step closer to being able to figure out the passwords for all your sites. And so it's extremely important for you to keep that secure.