What is Captcha?
The full form of captcha is Completely Automated Public Turing test to tell Computers and Humans Apart. This is basically a challenge to differentiate human and a computer software. A CAPTCHA is only solved by human being.
WordPress captcha plugin helps to stop spam attack and unwanted users from your WordPress website. The captcha plugins allow you to implement a super security captcha into your website forms like login form, registration form, password recovery form, comment form and any other form which you are using for website and protect against spam.
5 Best Free Captcha Plugins
SI CAPTCHA Anti-Spam – This is one of the best plugin for anti spamming. This plugin adds captcha to WordPress forms for comments, registration, lost password, log in or all.
In order to post comments or register, users will have to type in the code shown on the image. This adds security and prevents spam from automated bots. This plugin also compatible with WPMU, and BuddyPress compatible.
WP-reCAPTCHA – reCAPTCHA is also a best and most popular plugin. 
This plugin protects your site against spam, malicious registrations and other forms of attacks where computers try to disguise themselves as a human. reCAPTCHA comes in the form of a widget that you can easily add to your blog, forum, registration form, etc.
WP Captcha Bank –This is a powerful WordPress shield which allows you to protect your WordPress website from SPAM. 
You can display Captcha on Login Form, Registration Form, Comment Form, Admin Comment form, Contact Bank Form and Hide Captcha for registered users easily.Captcha Bank is loaded with more than 50+ settings including advance protection features such as Login Logs, Auto IP Block, Security Protections and much more.
Captcha – The Captcha plugin allows you to implement a super security captcha form into web forms. 
It protects your website from spam by means of math logic. You will not have to spend your precious time on annoying attempts to understand hard-to-read words, combinations of letters or pictures that make your eyes pop up. All you need is to do one of the three basic maths actions – add, subtract and multiply.
Captchas don’t work. And they are a pain for users. I’ve developed a proven technique, and it works in WP Contact forms and other PHP pages. It is free to use (open source), and I have found that form spam just stops dead when I implement it on web sites. All free, no ads, and I delete any emails I get requesting information. It’s all here brightverge.com and gocime.com. The technique works.,,,,,,,
Wonderful blog & good post. It is really helpful for me, awaiting for more new post. Keep Blogging!
White Hat World | White Hat Worlds | Search Engine Optimization | White Hat SEO VVV
nice list!
Hi Poonam,
Great post, however you may want to consider removing the mention of Sweet Captcha… It’s recently been linked to spammy Adware problems and has been removed from the WordPress repository (info: http://www.mikegracia.com/sweet-captcha-adware-warning-please-read/)
Just letting you know as I wouldn’t want people to see the recommendation here then go looking for it and run into problems 🙂
Mike
Great Article. Thanks for the info. Does anyone know where I can find a Registration form?
hi poonam ….i was searching article about CAPTCHA plugin and suddenly found your website.really nice post with attractive presentation.
Kcaptcha
The Kcaptcha plugin allows you to implement security captcha anywhere in website’s form. ksolves.com/products/product/captcha/
SFDC Lead Generation The plugin directly converts wordpress registered users to Salesforce leads. Now register a lead on Salesforce from wordpress automatically within fraction of seconds: http://ksolves.com/products/product/sfdc-lead-generation/
I have found an amazing captcha plugin for WordPress.
Kcaptcha
you can download from here products.ksolves.com/kcaptcha.php
Also they provide full support for free.
once i’d used WP-reCAPTCHA, but too bad for my theme, idon’t know why, then I did not use any anti span after reading this great plugin share article… Now i Use WP Captcha Bank – its so simple but so powerfull..
ms. Poonam..! thankyou for sharing this and will visit this site again next time…
Thanks. This is what i was searching for . Needed to open my comments section
You can definitely see your skills in the article you write.
The arena hopes for even more passionate writers like you who aren’t afraid to mention how they believe.
All the time go after your heart.
Very helpful post
Greetings! Quick question that’s totally off topic. Do you know how to make your site mobile friendly? My weblog looks weird when viewing from my iphone. I’m trying to find a theme or plugin that might be able to resolve this issue. If you have any recommendations, please share. Thanks!
Great list! Thanks for sharing!
Hello,
I have created a plugin ( https://wordpress.org/plugins/wp-forms-puzzle-captcha/ ) which adds captcha to login, registration of WordPress admin and CF7 plugin.