WordPress plugins are add-ons that provide additional functionality to a WordPress website. They can be installed directly from the WordPress plugin repository or by uploading a plugin’s ZIP file to a WordPress website. Plugins can add features such as contact forms, social media integration, security, SEO optimization, and much more.

Popular WordPress plugins include:

1. Yoast SEO: Search engine optimization (SEO) tool that helps with website ranking, metadata, and sitemaps.
2. Jetpack: All-in-one plugin with security, performance, and site management features.
3. WooCommerce: E-commerce plugin that allows the creation of online stores and selling of products.
4. Gravity Forms: Advanced form builder that allows the creation of contact forms, surveys, and more.
5. Elementor: Page builder that helps create custom page designs without writing code.
6. Contact Form 7: Simple contact form plugin for websites.
7. WP Super Cache: Caching plugin that improves website performance.
8. WPForms: Drag-and-drop form builder for creating custom forms.
9. All In One WP Security and Firewall: Security plugin that provides features like login protection, firewall, and malware scanning.
10. Slider Revolution: Plugin for creating animated sliders, carousels, and other visual elements.
11. The Events Calendar: Event management plugin for displaying events on a website.
12. Better WordPress Security: Another security plugin that provides features such as two-factor authentication, malware scanning, and more.
13. TablePress: Plugin for creating and managing tables on a website.
14. Redirection: Plugin for managing and tracking website redirections.
15. Autoptimize: Optimization plugin that helps improve website loading speed.
16. W3 Total Cache: Another caching plugin that provides website performance optimization.
17. WooCommerce PayPal Checkout: Adds PayPal checkout to a WooCommerce store.
18. Google XML Sitemaps: Plugin that generates an XML sitemap for better search engine optimization.
19. NextGEN Gallery: Plugin for managing image galleries on a website.
20. WP Fastest Cache: Another caching plugin for website performance optimization.
21. Jetpack Security: Security features add-on for Jetpack.
22. Broken Link Checker: Plugin that checks a website for broken links and helps fix them.
23. Polylang: Plugin for creating multilingual websites.
24. Akismet Anti-Spam: Plugin for blocking spam comments on a website.
25. Simple Custom CSS: Plugin for adding custom CSS styles to a website.

Note: This list is not ranked in any specific order and the popularity of these plugins may vary over time.