We build a lot of WP sites and have tried out just about every anti-spam plugin for our clients. None of them really hit the spot. Discovering Mollom has been an incredible breath of fresh air, here's a fantastic solution to one of the most annoying problems on the web. A definite 5 stars.
Mollom anti-spam requires some work to configure, because you need a key pair and a mollom account to set it up. It's also not always free, but it works really well.
Version 2.2 will not work on all themes for all post types since at least version 3.8 of WordPress. Out of the box, it will only work for the 'post' post type: not for pages and attachments. It also relies on single.php to be part of your theme, although this is not a requirement for themes. Furthermore, it needs your theme to use the hooks used inside comment_form(). If your theme uses a custom form (without calling comment_form), make sure it calls the right hooks or replace it with comment_form() if you want to use this plugin. Using these hooks is a requirement in all cases when you use Mollom.
On my Github fork of the Mollom plugin there is a version that supports posts, pages and attachments and also tries to use the appropriate template for those. There seems to be no attempt of the plugin authors to merge this, but when they do, I will come back here and rate it four stars.
I initially gave it four stars, because when it works, then it's pretty good and it works with the up to date core theme, but it is bad that it only supports the 'post' post type.
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