Limit rate of login attempts, including by way of cookies, for each IP. Fully customizable.

Everyone on WordPress should install this plug-in:http://wordpress.org/extend/plugins/limit-login-attempts/

WordPress › Limit Login Attempts « WordPress Plugins

wordpress.org

By default WordPress allows unlimited login attempts either through the login page or by sending special cookies. This allows passwords (or hashes) to be brute-force cracked with relative ease.