Jake

SpotBot was created for the purpose of controlling spam before it clutters up your server. Gone are the days of constant comments about Gucci bags in broken English. SpotBot not only stops spam from comments, but also stops bots from creating fake accounts or even logging in, but the fun doesn’t stop there! A custom action has been built in so you can add it into any template to stop access to that page!
<h4>So how does it work?</h4>
Every task that starts or ends in WordPress starts with an action! SpotBot utilizes these actions, to gain control over who has access to your site! But how can it tell who’s bad and who’s good? SpotBot uses a service from BotScout to determine who the baddies are. They have allowed public access to their database through an API that SpotBot utilizes. This API access is limited though, and only allows around 300 queries to it in any 24-hour period. You could pay BotScout for more access as they’re fairly cheap, but to help them keep their costs down and help you have control over your visitors, all IP’s are logged to your database. Each time a returning IP is found, it checks the cache first and if nothing is found checks BotScout for more information. It’s as simple as that, SpotBot can be a set-it-and-forget-it plugin, but it can be more than that. Have a problem with a particular user that keeps making user accounts? Flag them as a bot! The harder they try, the more flags they generate and the higher they go on the bot score! SpotBot comes with a built in search function to make finding those troublesome bots easier than ever. Don’t want to save the IP in a list of trouble makers? Make a note of them in each IPs note section for easy searching. Can only remember the first 3 digits of the IP address? Search for it!

SpotBot

SpotBot aids in controlling spam before it clutters up your server. It stops bots in their tracks before they have a chance to post or force post.

sdesalas

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.
This allows for a minimum sanity check and yet should remove almost all spam comments 
without the need to sign up to any third party APIs.
The plugin has been tested in real world conditions for around 12 months. It will not block 
real comments and it is pretty effective against spam, however it only uses a very simple 
algorithm and for that reason some spam comments may still make it through.

WP Simple SpamCheck

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.

SpotBot vs WP Simple SpamCheck