About Plugin
With help of plugin you can extend functionality into WordPress cms that already exit in WordPress core. Plugins then offer custom functions and features so that each user can adjust their site to their specific needs.
1) Add Meta Tags
Add basic meta tags and also support Opengraph, Schema.org Microdata, Twitter Cards and Dublin Core metadata to optimize your web site for better SEO.
Official Download Link
2)Advanced Twitter Followers Widget
Advanced Twitter Followers is free twitter followers display widget. Using this widget you can show twitter followers on your wordpress website.
Official Download Link
3)Akismet
Akismet checks your comments against the Akismet Web service to see if they look like spam or not.
Official Download Link
4)NinjaFirewall (WP edition)
NinjaFirewall (WP edition) is a true Web Application Firewall. Although it can be installed and configured just like a plugin, it is a stand-alone firewall that sits in front of WordPress.
This plugin help to protect wordpress site from different type of attack for example:-remote & local file inclusions, code execution, uploads, MySQL injections, brute-force scanners, XSS and many other threats.
It allows any blog administrator to benefit from very advanced and powerful security features that usually not available at the WordPress , but only in security applications such as the for Apache ModSecurity module or the for PHPSuhosin extension
Official Website
Official Download Link