I run several online forums, and one forum is in the health field and it is constantly under assault by people and bots who want to spam it with ads for cheap pills. No amount of CAPTCHA or skill-testing questions can stop these determined individuals, so I've resorted to approving every new registration.
Most registrations from the spammers are easy to spot because they have user names with a number in it. For example, "johnsmith858". Then there are the ones with no imagination whatsoever who use names like "buycheappillsonline". Occasionally, however, I get a registered user name that seems legit, so I have to decide to approve or not approve. What if I approve it and it's a spammer? What if I delete it and it's a real person who wanted to use the board? Rather than rely on my gut (something I used to do) I now use StopForumSpam.com to check whether the person is a spammer or not. So far, it has been 100% on target with all my "is it?" or "isn't it?" registrations.
All you have to do is take either the user name, IP, or email address of the new user registration and plug it into the StopForumSpam.com website and you'll get an answer in a few milliseconds.
Fantastic utility that takes the guesswork out of new forum registrations. Here's the link again: StopForumSpam.com.