http://wordpress.org/extend/plugins/wp-hashcash/
WP-HashCash implements the hashcash approach to spam prevention as a WordPress plugin.
http://www.hashcash.org/
Hashcash is an anti-spam technology which requires the sender to compute a partial hash collision (which takes a predictable amount of time) in order to "pay" for their email. On the flipside, it's very easy to verify the hash.