Generally WordPress website owners use CAPTCHA plugins for protecting their site from Spam Bots. This tutorial explains you working of a free WordPress plugin you can use for protecting your WordPress installation from Spam Bots without installing any type of captcha plugin. Keypic.Com is an online service that provides you with free plugins now only for WordPress but for other Opensource blogging platforms like Drupal and next month (as scheduled) they would be launching plugins for Joomla, php. More plugins would be available in the near future for MediaWiki, Typo3, vBulletin. For more information on plugins visit here.