Comprehensive and Easy-To-Use WordPress Security - Comes With Business Grade Support, with no "premium" restrictions
The Whitelist plugin enables you to open your website to a set of users and blocks everyone else. You can enter users by their email address into the
Block users using blacklists domain from registering to your WordPress site.
The AVH First Defense Against Spam plugin gives you the ability to block spammers before any content is served.
The plugin Limit Attempts allows you to limit rate of login attempts by the ip, and create whitelist and blacklist.
Gatekeeper allows administrators to take a WordPress site offline while leaving it accessible to authorized users for maintenance, testing, etc.
Restricts registration to email addresses listed within the options file. Assumes WP native registration is turned off.
This plugin adds creates a whitelist of shortcode tags that can be applied to comments, bbPress and BuddyPress.
Grants or denies access to a list of IP addresses
Enables the use of Albus whitelisting plugin for Bukkit (Minecraft) on WordPress installations
Adds Whitelist and Blacklist options for Trackbacks and Pingbacks
Limit access to the site to those with a cookie token. Visitors without the cookie see a customisable "coming soon" style of page.
Limits access to the site to people on an allowed list of IP addresses.