Misc Anti Spam adds a couple of minor defences to your forum to stop miscreants getting through. They're no magic bullet but every little helps.
* A time gate on registration to prevent overly quick registrations from being accepted. * Empty field appearing on all places a CAPTCHA would (has two variants, a hidden field that should stay empty, and a field that will be visible that a user has to actively empty out, either way the field needs to be empty before the form will be accepted)
Most of the time, most users won't see anything because it's supposed to work in the background. But if they do get shown the must-be-empty field, the attached screenshot is what they'll see.