On most websites there are some kinds of forms that require the submission of data. Be it the simple contact form or the commenting functionality for blog entries or other content types. If so you will need a way to protect these forms from spam.
The key module to do so is the Drupal CAPTCHA module. The purpose of CAPTCHA is to block form submissions by spambots, which are automated scripts that post spam content everywhere they can. The CAPTCHA module provides this feature to virtually any user facing web form on a Drupal site. As per default, the Drupal CAPTCHA module offers you image and math challenges for the different forms.
This is probably the most important extension. It uses the reCaptcha web service from Google to improve the CAPTCHA system and protect email addresses. reCaptcha helps power massive-scale online collaboration. reCaptcha Mail hide helps you protect your inbox by asking people to solve a reCaptchabefore they can view your email address. A slight disadvantage of this module might be, that you have to register, but this can be done within seconds. Unfortunately the Drupal module reCaptcha does not yet support the “No Captcha reCaptcha”, which is a great step regarding security and usability.
A lovely extension to make CAPCHA more personal/individual by adding a question as challenge. Captcha Riddle is a sub module of Captcha that lets site administrators creates their own questions to foil automated spam bots.
Uses the Google reCAPTCHA web service to improve the CAPTCHA system and protect email addresses. This module is inspired by the reCAPTCHA module, but uses the new Google No Captcha reCaptcha API. On websites using this new API, a significant number of users will be able to securely and easily verify they’re human without actually having to solve a CAPTCHA. Instead, with just a single click, they’ll confirm they are not a robot. While the new reCAPTCHA API may sound simple, there is a high degree of sophistication behind that modest checkbox. CAPTCHAs have long relied on the inability of robots to solve distorted text. However, Google's research recently showed that today’s Artificial Intelligence technology can solve even the most difficult variant of distorted text at 99.8% accuracy. Thus distorted text, on its own, is no longer a dependable test.
Mollom is a freemium (free up to a certain level) anti-spam measure created by Dries, the creator of Drupal. You can configure it to only present the captcha after scanning the user's input and figuring out that it might be spam. While it apparently works quite well, I've never been a fan of it. I find it ugly and I find that sites with it keep asking me to fill it out. Perhaps the comments I leave on the internet are all spam.
1.Allows boost caching for Drupal Pages.
2.Allows forms with CAPTCHA to be cached even after the CAPTCHA session expires
3.Useful when forms are loaded in blocks
4.Preventing form submission