Captcha is known for its efficiency in ensuring proper use of websites and online application by validating that the user is human and not a bot or some other automatic machine.
This article lists some of the security applications / uses of CAPTCHA:
Comments spam prevention
Comments spam is used in blogs and forums mostly for raising search engine ranking of websites. A “spammer” who wants to get some free traffic to his website might get into a variety of blogs or forums and add comments with textual (using keywords as the anchor) links leading to his website.
The advantage of using captcha in blogs and forums adding comments option is that it can block automatic machines that can do this kind of spamming. Bots don’t know how to read captcha and thus cannot pass it.
Online Surveys and Polls
When you want to trust the results of an online poll and be sure that only human votes are counted – use Captcha. This way you’ll ensure that only human will take a part in the poll and at the same time prevent the possibility of biased results (by bots).
Spam blockers / Protecting Email Addresses from spam scrapers
Captcha is also known as a great spam blocker against spammers who search the web for e-mail addresses. You want to allow your users to send you emails but don’t want to get spammed by bots? All you have to do is to ask your website visitors to solve a confusing Captcha before you show your e-mail.
Just the same, the CAPTCHA test can be used to protect from massive registration to free email accounts.
Search Engine Bots
Have some confidential pages on your website that you don’t want the search engine bots to reveal (and index)? Use Captcha to prevent all kinds of bots, including those of the search engines, to get to them.
Free anti spam
Lots of companies wish to accept mails only when they are 100% sure that the e-mail sender is not an automated machine. One of the easiest ways to get this is by using Captcha as a spam filter.
