The ciphercoin premium plugin adds some features to the wp limit login attempts plugin. At the moment, the extra included functionality includes:

Unbreakable captcha: Commonly WordPress security breaks by using WordPress brute force tools like wpbf. An unbreakable captcha system will stop automated access.  We added a random background in the captcha. So captcha breaking is not possible.

Demo: http://CipherCoin.com/wp-admin/

Unlimited installation license per user

You can use the pro version on unlimited websites.