htn_antispam is a simple plugin to prevent your textpattern blog from spamattacks. The plugin generates a inputfield in your commentform, which should be hidden through css, so most of your visitors will not recognize it anyway.
The plugin checks this field when a comment has been send. If the field is not empty (it should be empty if nobody sees it), the plugin will generate an error-message and the comment will not be saved. Because every spambot is trained to fill out every input-field he sees in a form, nearly every bot will go into this trap.
The second function of the plugin is to check the comment for BBCode, which is used by spambots nearly every time they throw a comment on your site. This feature also works without the TXP-Tag in your comment form, so the hidden inputfield-technique is only optional.