If you would like to use Captcha on your forms, you could use this plugin. BUT!!!:
You need to provide your Private and Public ReCaptacha key on the settings page of this plugin. Since I did this, I've got a lot of spam via the comment field on my articles, which is also using ReCaptcha.
I am not sure and aware how and why, but it starts shortly after I safed that infomration on this setting page and it ends, as I have disabled the plugin and changed the key for ReCaptcha. So the issues must be related to that plugin and I have it uninstalled, because of the missing trust.