Free Image-Based Captcha: VidoopCAPTCHA
VidoopCAPTCHA is a free verification solution that works image-based which is unusual compared to the widely-used text-based ones.
It aims to be a more user-friendly solution as text-based captchas can sometimes be so hard to read for humans besides bots.
VidoopCaptcha is a hosted service & have plugins for WordPress, Ruby on Rails, and Drupal. Also, there are libraries for programming languages such as PHP, Python, and .NET.
It offers some customization options like category, number of grid squares, color & grid size. For a demo of this security resource, click here.