Some CAPTCHA plugins for WordPress contain hidden backlinks that give “link juice” to their own pages. Below I have tested some to see which ones are “nofollow” either containing nofollow links or no links at all. As well as covered a bit of the basic functionality. The ones that failed to pass the backlink test are listed below the article. You can also check out my list of backlink free web analytics services.
More will be added to the list over time.
Nofollow
Peter’s Custom Anti-Spam
– Easy to read CAPTCHA where you can select the different words and fonts used. Also has a feature to disable trackbacks & pingbacks. If you click on the produced image, it will open a .wav file which spells out the letters and numbers.
Eazy CF Captcha
– simple math captcha, easy for bots to solve. can change the text for it only.
Captcha
– a math captcha shown with words & numbers. especially easy to break for spam bots because it’s written in plain text. can be added to registration form also.
Checkbox Captcha
– adds a simple checkbox to post/page commenting that you need to deselect to enable the submit button. obviously not a strong enough protection on it’s own.
visualCaptcha
– an easy captcha where you have to select the right image based on a word. alternatively you can listen to the captcha, where you answer to a simple question math or other.
NuCaptcha
– visual javascript generated captcha, with moving text. no links at all are generated. registration of a free “NuCaptcha Account” required. seems there are quite some spam bots that getting through. no longer listed on wordpress.com
Dofollow
Sweet Captcha
– a sweet, graphical captcha that adds dofollow link to every post. removal or nofollowing the link has been made difficult on purpose.
I personally like using “Conditional Capchta” for my site Bilqees Kenchi because then it only shows to spammers. Less annoyance but the same amount of protection.