Free spam IP blocker according to public DNSBL bases.
Block unwanted users from registering to your WordPress site with custom defined blacklist and whitelist rules.
Botnet identifier using public DNSBL bases and a hardcoded whitelist/blacklist.
Check if a visitor is listed on httpBL.abuse.ch, see http://dnsbl.abuse.ch/faq.php#about_httpbl for more information.
Tornevall Networks DNS Blacklist support for WordPress