Reward your readers by automatically placing a link to their last blog post at the end of their comment. Encourage a community and discover new posts.
Todd Lahman's comment spam blocking plugin that blocks automated spam with zero false positives.
SiteBrains blocks spam posts using a completely configurable set of rules. Both interactively as the user types and on our servers upon submission.
Checks where visitors come from and gives them a warning if they are likely to be comment spammers. For DoFollow and exDoFollow blogs.