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 breaking by using wordpress brute force tools like wpbf. Unbreakable captcha system will stop automated access. We added random background in captcha. So captcha breaking is not possible.
Demo : http://CipherCoin.com/wp-admin/
Unlimited installation licence per user
You can use pro version on unlimited websites.