Sophos recently released a report on password security that indicates that only 19% of people use multiple passwords to access different websites (based on an online survey of 676 people). From the remainder, 33% use one password to access all websites and 48% use a few different passwords.
It is recommended that users assess their passwords for strength (read more about that here) and use different passwords to access different sensitive accounts. Doing so will help users protect their personal and corporate data. There are more advanced password strategies you can employ if you want an added measure of security - these can include the use of tools like PassSafe. Here's a video that Sophos put together talking about password security:
As Sophos notes, password security should not be overlooked. Far too many people stick with dictionary words, or simple passwords such as "1234". These passwords are easily guessed by hackers and can be used to exploit a computer network. For example, one Conficker-infected computer can be a risk to a whole network, with the worm using 200 common passwords to try to spread.