The hashcash algorithm
Yeah, it has a weird name, but it's one of the most interesting (and effective) anti-spam algorithms out there: Hashcash. How does it defeat spam without the end-user even knowing a spam check is taking place? Read on.
I think Hashcash is such I good idea, I'm going to give it a test run on this site using the WP Hashcash plugin for WordPress. Hopefully I won't have to manually deal with spam for a long time to come. :)