What is the best way to check the strength of a password?

Question

What is the best way of ensuring that a user supplied password is a strong password in a registration or change password form?

One idea I had (in python)

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Answer 1

In addition to the standard approach of mixing alpha,numeric and symbols, I noticed when I registered with MyOpenId last week, the password checker tells you if your password is based on a dictionary word, even if you add numbers or replace alphas with similar numbers (using zero instead of 'o', '1' instead of 'i', etc.).

I was quite impressed.