reCAPTCHA has the highest security standards. Many other implementations of CAPTCHAs use undistorted text, or text with only minor distortions. These implementations are vulnerable to simple automated attacks. Others, such as MAPTCHA, consist of asking text-based arithmetic questions like "what is 1+1". These can be trivially broken by an attacker.
To protect your site, reCAPTCHA uses two layers of security when generating images. It starts with images that can't be read by computers, and then distorts them even more:
reCAPTCHA is a Web service. That means that all the images are generated and graded by our servers. In addition to the convenience that this provides (you don't have to run costly image generation scripts on your own servers), this also provides an extra level of protection: our CAPTCHAs can be automatically updated whenever a security vulnerability is found. For example, if somebody writes a program that can read our distorted images, we can add more distortions in very little time, and without Web masters having to change anything on their side. This is significantly more secure (and convenient) than having to re-install a CAPTCHA every time a vulnerability is found.
IP Address Detection
Our service also includes IP address filtering and detection. If we determine that a given IP address is successfully solving too many CAPTCHAs in a certain period of time, the address is immediately flagged for review. In addition, by providing CAPTCHA services to many customers we obtain a global view of spamming attacks, allowing us to react quickly to security threats.