WP-Hashcash Version 4.5
Client-side javascript blocks all spam bots. XHTML 1.1 compliant.

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. If the javascript check fails, WP Hashcash now gives you three options; it can either put the comment into moderation (default), put the comment in the akismet queue, or delete it.

Features

Blocks all comment spam, but not real comments
Also prevents most trackback / pingback spam
Widget support to display spam statistics and edit the configuration
Works with IE, Firefox, and Safari
100% standards compliant XHTML 1.1
Tested with Wordpres 2.6, Firefox 2, Safari 3, and IE 7
Akismet compatibility
Limitations

WP Hashcash relies on the presence of two hooks in your theme, wp_head and comment_form. If your theme doesn't include these actions, you will need to add them immediately before the end head and end form tags respectively.

Download: