Text Captcha uses riddles and math to make sure your posters are real instead of computers.
It doesn't take much to prove that computers are idiots, but most captchas go too far. Captchas based on graphics don't work with blind people yet they dominate comment forums. This is a logical captcha that uses math and simple riddles to weed out the machines. This plugin comes with 10 predefined questions and the ability to add more. The math part isn't too hard and can be disabled if you're dealing with people who don't like math. This plugin relies on php sessions to store information.