The word Captcha stands for 'Completely Automated Public Turing test to tell Computers and Humans Apart'. In plain English, a Captcha detects whether or not someone visiting your site is human or an annoying, spammy bot. The user is asked to prove his/her existence by either typing or responding to images.
This is really important, in terms of maintaining online security, because it protects you from spam and privacy invasions. So you really should consider adding a Captcha plugin. Here are five of the best on the market right now. Some are free, while others will set you back a few dollars. Some are straight-forward, while others are innovative and fun. All of them are highly likely to save you time and give you peace of mind.
myCaptcha is easy to install and features a clean design. There are no fewer than 37 options for customisation, so you can direct it to suit your specific needs. That's whether you want to customise fonts, patterns, backgrounds or the dictionary. There are also seven image filters. It costs a one-off payment of US$17.
Like myCaptcha, Google Captcha is straightforward when it comes to installation. But it is not so flexible as far as display goes. Where myCaptcha - Powerful Human Verificator is extremely customisable, Google Captcha offers just four standard themes. The good news, though, is that it is absolutely free and yet very reliable.
UPDATE: 22/06/15 - Original review removed, replaced with below.
This doesn't have the bright, image-based element that gives Sweet Captcha its edge, but it is kind of fun, because it asks visitors to write answers to given sums. You can choose whether to display these as numbers or words and select a particular mathematical operation. Free Math Capthca has been translated into ten languages and, as the name suggests, won't cost you a cent.
Here's another in the fun category. Like Sweet Captcha, it relies on a drag and drop mechanism. Three sizes (small, medium and large) and four models are available, so you can choose which best suits your set-up. Like myCaptcha, it'll set you back US$17.