Many will use the popular Askimet and others but it's good to add a tool more to combat the annoying spam .
It works in coordination with Askimet or without ; You have options to manage from the panel; and allows if a comment passes (unlikely, but possible), send it to the moderation queue. also works against unwanted trackbacks (references)
Although I have not checked it, nor is it in the available documentation of the plugin; would encourage me to say that it helps with the WordPress vulnerability 2.3.3 due to its operation.
It is 100% compatible with XHTML standards, WordPress 2.3.3 and 2.5; Firefox 2.x, IE 7 (I tested it on 6 and it works fine), and Safari.
Since I used it (alone, without Askimet) I had no problems; and so other users on the plugin page claim the same fate.
Download Hashcash (latest version) 4.1
Please download the plugin and unzip. Then copy the file wp-hashcash.php to wp-content / plugins (directly to the plugins folder).
Activate the plugin. Optionally you can use Widget sidebar, but its function is only to offer public statistics (nothing technical).
Within the administration panel, you can configure the options, entering in: ] Options> WP Hashcash .
Does not require you to modify any code, it is automatically integrated (using the presence of 'wp_head' and ' comment_form  ' included in most of the themes.)