reCAPTCHA is an anti-spam technology from Google.One of the best anti spam plugin for wordpress. It uses a captcha to prevent comment, registration, and email spam.
To add reCAPTCHA to your WordPress blog
1. Download reCAPTCHA WordPress plugin from this link.
2. Unzip and upload the recaptcha folder into your WordPress wp-content/plugins directory.
3. Login to wp-admin. In your admin dashboard click plugins and activate the WP-reCAPTCHA wordpress plugin.