Update: Brute Protect is now built into Automattics Jetpack plugin.
However, as of 2018 – I would recommend using Wordfence. and I have updated all my sites to use it.

If you have a standalone WordPress installation, I recommend adding the Brute Protect plugin. Brute Protect is designed to reduce the chance of your WordPress site falling victim of a ‘Brute Force Attack‘. This is where a nefarious computer system (a malicious bot) continually tries to break into your WordPress account. More >