This plugin provides you an easy captcha without javascript, without images, without math. Only a few of CSS and a few of server side. 
An hidden text input is placed in comment form a human cannot see it so a human cannot compile it, a spambot otherwise see the text input and is forced to compile it. 
The plugin check the existence of this text input and check if it has a value or not. It drops all comment with a value in the text input as spam. 
Really simple and powerfull antispam system. Low CPU usage and no access to DB.

Hiddy captcha 1.5.1

This plugin provides you an easy captcha without javascript, without images, without math. Only a few of CSS and a few of server side.

Todd Nestor

Hercules Recaptcha uses the latest Google Recaptcha API to more accurately determine if users are bots or not. 
If the user is not logged in it will display a Recaptcha for the user to fill out in the comment form. If the user 
disables javascript and is not logged in then comments will fail to submit.
The Recaptcha is also added to the registration page for both multisite setups and single blogs. There are options for 
having it show up on comments and/or the registration page, as well as options for its position on the comment form, and 
which style (Google gives only two options, dark or light).

Hercules Recaptcha

Hercules Recaptcha adds a Recaptcha to the comment form for non-logged in users.  It uses the latest Recaptcha API.

Hiddy captcha 1.5.1 vs Hercules Recaptcha