Skt NURCaptcha

If your Blog allows new subscribers to register via the registration option at the Login page, this plugin may be useful to you. It includes a reCaptcha block to the register form, so you get rid of spambots. To use it you have to sign up for (free) public and private keys at reCAPTCHA API Signup Page. Version 3 added extra security by querying antispam databases for known ip and email of spammers, so you get rid of them even if they break the reCaptcha challenge by solving it as real persons.