How Secure is My Password lets you know just that
That's a handy set of rules to keep in mind, but How Secure is My Password helps us understand why they're important.
It's basically like a full-screen version of one of those password-strength meters websites sometimes use. But instead of showing you a bar going from "weak" to "strong", it shows you an estimation of how long your password would take to crack. That's a much more visceral way to understand why your password is strong.
For example, when I entered "rabbit", it came back with "your password is one of the 500 most common passwords. It could be cracked almost instantly". "rabbit5" would take two hours, "$rabbit5" would take 38 days, and "$rabbitZ5" would take 237 years. It's quite enlightening to see what a difference three simple characters can make.