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I'm looking for a free captcha service for a html/css/php site. reCAPTCHA seems to be very famous. Is it effective? Are there maybe other good captcha services which are available for free?

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up vote 4 down vote accepted

There are a lot of free captcha services, but there are also a lot of new tricks you can do to avoid using captcha's altogether.

One very popular approach is to add a hidden field inside your form. On your backend, you can check whether the hidden field is filled in. A bot would fill the hidden field, a user not. If you catch a hidden field that is filled in you know you can discard the message.

Another way is to work with your php session and special keys. Generate a key fill it in the a hidden field of the form, and store it in a session. When the user submits a form, check if the value submitted in the hidden field is the same as in the session. If a form is submitted, and there is no session key stored you know it is a bot.

And so there are a lot of small tricks you can apply to catch bots. You can also mix up a few.

To catch spam you can also use a tool called akismet. It does a pretty good job of catching spam comments.

If you can, don't bother your visitors with a captcha, there are a lot of good solutions out there. But if you really have to, reCaptcha is a pretty good solution, I have used it for a while with good results, it's only a pain for your visitors to fill in. I hate it when I need to fill in a reCaptcha.

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Great points. It's worth noting your method of storing a session somewhat needlessly forces a cookie on a user (which really isn't a big deal, unless you're dealing with a bunch of privacy advocates or something similar). –  Nick Q. Nov 9 '11 at 21:12
    
Ah that's true, never thought of that one. Well that is a good point to remember. –  Saif Bechan Nov 9 '11 at 21:27
    
Actually adding extra cookies might soon become an issue due to new EU privacy laws announced earlier in the year - see ico.gov.uk/~/media/documents/library/Privacy_and_electronic/…. Regarding captchas it's worth adding that recaptcha is great because it helps with the digitisation of books. –  Brighty Nov 10 '11 at 9:21
    
Along the same lines, the Growmap Anti-spam Plugin for WordPress uses a number of methods that thwart automated spam very well plus it degrades well. You might want to look it up and see if you can use it or borrow ideas from it. –  jfrankcarr Nov 10 '11 at 15:19
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