What Are Common Password Checks?

robra asks: "For a Web site I am writing using Java servlet technology I need to ensure that users do not pick "bad" passwords. I know there are many C programs like Crack which try to break encrypted passwords and I could use one of those to see to it that users don't pick a password that can be broken. However, platform independence is a big issue and so I would like to stick to Java code only. Does anyone know where I can find some Java code to check for bad passwords?" In the interest of making this a little more open, what kind of tests do most password checking algorithms perform to insure a password isn't too easy to crack?