The very popular WP Hashcash plugin for WordPress has been modified to work on the WordPress MU signup page.
This is the first release of the code so handle with care. Grab the latest version (version 4.2 as of this moment) from the download page. Unzip it and copy wp-hashcash.php into wp-content/mu-plugins/ and visit “Site Admin” -> “WordPress Hashcash” to confirm it’s working.
Keep an eye on the stats counter on the admin page. I want to hear how well this works on your site!