Defensio is a comment spam prevention service, similar to Akismet. It's from a company based in Montreal.
Spam filtering service with plugins for major blogging / CMS engines. Free account passes up to 100 comments per day, then pro accounts for €30 /month or $3'600 a year.
The comment referrers plugin stores the HTTP referrer field when a comment is submitted and then adds it to the comment in the admin / email view. Helps to identify spam.
WP-HashCash implements the hashcash approach to spam prevention as a WordPress plugin.