Previously, I used "Joo ReCaptcha". That is very a good plug-in. But ReCaptcha got a bug in IE. When its captcha text field will disappear when I use jQuery.blur() or focus(). I used superfish menu. It used these two jQuery methods a lot. When I show the sub-menu, then captcha text field will disappear. I have spent a day to fix it but I cannot find a solution or work around.
Thus, I gave it up. I used another Captcha Plug-in which is not using ReCaptcha. That is OSOLCaptcha. This is very similar with ReCaptcha, but it can convert captcha text to speech. This may be some accessibility issue. Besides of this, that is very similar with Joo ReCaptcha. You just need to install the plug-in, then it will works in Contact form, login and register form. That is good.

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Good captcha plug-in in Joomla