Akismet is best spam detector plugin but all comments made by spam bots end up in your WordPress Spams folder. This tutorial is about a light weight WordPress plugin that you can easily use with Akismet and increase a step for spambots when posting comments in your WordPress blog.
Growmap Anti Spambot Plugin is a free WordPress plugin. After activation this plugin simply adds a client side generated checkbox in WordPress comment forms. This check box asks your commenters to confirm that they are not a spambot, hence they easily check it which is a much less trouble than filling wired captcha. Check box is genereated via client side javascript that is invisible for bots and it should stop 99% of all automated spam bot comments. The plugin provides you with some customization options that you can visit from its admin area settings page.