NoCaptcha automatically prevents unsolicited form submissions without the need for those ugly and unfriendly captcha fields.
Employs methods derived by Eugene Kerner of eKerner.com and later employed in Google website forms.
To configure: Simply set 'Default Captcha' to 'Captcha - NoCaptcha' via "Administration -> Global Configuration -> Site tab.
Where other extensions allow you to set the Captcha method, assure those are set to 'Default' or 'Captcha - NoCaptcha'.
You can debug and fine-tune NoCaptcha via her plugin settings.
Developed by eKerner.com - technical developer
Disclaimer: NoCaptcha greatly reduces the number of unsolicited form submissions received, however - like any Captcha method - it cannot prevent human spammers, nor elaborate and purpose designed Captcha bypass apps (like 'AutoCaptcha').
The developer (me) makes no promises and takes no responsibility for the malicious content posts - or other abilities - of spammers and hackers.
This software is provided simply to aide you in your fight against spammers and malicious processes.
I use NoCaptcha on my own sites and it filters out most malicious attacks while not annoying my website users with Captcha texts that they cant read :)