by Razvan Aldea on WordPress.org
This plugin adds the reCaptcha form to the WordPress login form, recover password form, register form and comment form.

Back-end UI
A very useful plugin for everyone using WordPress. Adds reCaptcha security to the WordPress login form, register form and comment form. This plugin could help your blog get rid of a lot of spam comments or brute-force attacks.
Nothing gets passed it if the reCaptcha doesn’t validate.
A few notes about the plugin: