WP No-Bot Question is a WordPress plugin by edwardw which adds a question to the comment form that blocks most automated spam-bots by making them answer a common sense question. For optimal effectiveness, you might want to consider changing the question periodically so that the spambots can’t “memorize” the question and answer.
Multiple answers can be added to account for variances in answers. (e.g. “five”, “Five”, “5”)
Plugin homepage: http://www.compdigitec.com/apps/wpnobot