These security plugins were 100% tested by me and are suggested to all wp users.

Limit Login Attempts:
This plug in is a very important and efficient security plug in, limiting your plug in attempts for users. you can limit log in attempts, decide a email notification when someone reaches limit number you settle.

Ask Apache :
Adds some serious password protection to your WordPress Blog. Not only does it protect your wp-admin directory, but also your wp-includes, wp-content, plugins, etc. plugins as well.

Database Backup :
WordPress database backup creates backups of your core WordPress tables as well as other tables of your choice in the same database.

Members Only :
Members Only is a WordPress plugin that allows you to make your blog only viewable to visitors that are logged in. I don’t suggest it for startup bloggers.

Allow Categories :
Allow Categories is a plugin that allows you to permission different user to view only specific categories within your wordpress blog. Suggest only for real traffic blogs, and only to some categories.