Downloading: akismet_plugin-1.8.zip Akismet
You'll need a "Wordpress API key", which you can get by signing in on http://wordpress.com/.
The plugin requires b2evolution version 1.8 or later.
List of $_SERVER keys, that should additionally get sent with the comment data. The default should be safe to not include sensitive data.
The Akismet plugin now also protects the message form.