Simple WP antispam
It uses 6 methods:
a) honeypot field "nick", it is hidden by CSS - only bots will fill it
b) block comments with BB code [url=...]
c) HTTPBL (DNSBL) from http://www.projecthoneypot.org - you need API key
d) Block comment with common bad words
e) Block direct POST requests (no referer)
f) Swap regular comment textarea with honeypot field