Last August, Troy Hunt launched a little feature within Have I Been Pwned (HIBP) called Pwned Passwords. This was a list of 320 million passwords from a range of different data breaches which organisations could use to better protect their own systems. V2 just got released and there's Now 501,636,842 Pwned Passwords, but the biggest improvement is elsewhere. He got a lot of feedback from V1 along the lines of "simply blocking 320M passwords is a usability nightmare". Blocking half a billion, even more so.
In V2, every single password has a count next to it. What this means is that next to "abc123" you'll see 2,670,319 - that's how many times it appeared in my data sources. Having visibility to the prevalence means, for example, you might outright block every password that's appeared 100 times or more and force the user to choose another one (there are 1,858,690 of those in the data set), strongly recommend they choose a different password where it's appeared between 20 and 99 times (there's a further 9,985,150 of those), and merely flag the record if it's in the source data less than 20 times. Of course, the password "acl567" may well be deemed too weak by the requirements of the site even without Pwned Passwords so this is by no means the only test a site should apply.