Some of the popular plugins are:

1. Wordfence Security – This plugin is one of the most popular and highly rated security plugins for WordPress. It offers a comprehensive security solution with features such as a firewall, malware scan, and security alerts. The plugin also provides two-factor authentication to enhance security.
2. Jetpack Security – Jetpack is a multi-feature plugin that includes security features such as malware scanning, brute force protection, and a firewall. It also includes daily site backups, which can be used to restore the site in case of a security breach.
3. iThemes Security – iThemes Security is another free plugin that provides an array of security features, including a firewall, malware scan, and two-factor authentication. It also includes a feature to detect and block malicious login attempts and hide sensitive information to prevent hackers from exploiting known vulnerabilities.
4. Security Ninja – Security Ninja is a comprehensive security solution that includes a malware scanner, firewall, and security optimization features. The plugin also offers a security checkup, which scans your site for potential security risks and provides recommendations to fix them. It also has a feature to automatically update WordPress core, plugins, and themes.
5. All In One WP Security & Firewall – All In One WP Security & Firewall is a free plugin that provides comprehensive security features such as a firewall, malware scanner, and two-factor authentication. It also includes a feature to hide the WordPress login page to prevent hackers from gaining access to your site.

Summary:

In conclusion, these are just a few of the best free WordPress security plugins available to help secure your site. Remember that no plugin can guarantee 100% protection against all security threats. The best approach to ensure the security of your site is to keep it up-to-date and regularly monitor it for potential security risks.