For much of the modern world, all that stands between you and utter disaster is a password. Lots of online accounts depend on an email address as a user name, so if somebody's after your stuff, they're halfway there just by virtue of knowing that address. That means that choosing a good password is even more important.
The best password is the password that has the least meaning in the real world. Your pet's name, dictionary words, the word "password"? Not such good choices. Passwords that include both upper- and lower-case alphabetic characters, numbers, and even punctuation marks are better. And you get bonus points for making them extra long as well. So how do you know if your password is a good one?
Check out The Password Meter. This free online service takes a look at your password and lets you know how it measures up. Sure, there are no "password police" out there to really say that you've made good choices or bad, but these guys will point out places where you could improve things (no repeated characters) and where you've made good choices (including at least three of the four character types listed above). One drawback to this tool is that it caps you at a max of 16 characters for passwords that it tests. The security of your passwords will increase dramatically as the number of characters increases.
The Password Meter is a free service. If you've got a browser and care about security, you should be good to go.