From Habari Project
Spamcheck is a feature which allows comments to be evaluated to determine whether they are spam or not.
To implement spamcheck, you need to check comments before they are inserted into the site's database to assess the probability that they are spam, rather than valid comments. The evaluation may utilize internal methods or the facilities of an external service. Comments that are determined to have a high likelihood of being spam should be marked as spam. Optionally, such spam may be deleted immediately.
The following plugins provide the spamcheck feature: