WP Hashcash is an antispam plugin that eradicates comment spam on Wordpress blogs. It works because your visitors must use obfuscated javascript to submit a proof-of-work that indicates they opened your website in a web browser, not a robot.

He includes the code that he needed to change to get the widget part of the plugin up and working correctly. You can find out more about this plugin from its page on Elliot Back's blog.