Unspam makes it harder for spammers to automatedly send spam to your blog by changing the names of the fields in the comment forms.
There is currently no configuration available on this plugin, just install it and activate it and it will start protecting your blog.
This plugin is special in that false positives should never really occur with the exception of the small possibility that a human visitor might open a page just before midnight then submit a comment just after midnight. Their comment would be rejected as spam. I expect to put in a feature to prevent even this from occurring in the near future.