Wow, just installed this plugin a couple of weeks ago on a completely un-used WordPress site. Since then, it has caught so many issues. Including 2 recent hack-attacks where someone was constantly hammering it login attempts, and it automatically banned their IP from accessing login page or password reminder page, and sent me warning emails about it.
Just goes to show that even your most under-used WordPress sites are being hammered all the time. So install this on every one of your WordPress sites. And for your main sites, get the pro version.