Suffice it to say that WordPress is incredibly popular now that it has surpassed 25% in the worldwide marketshare. And one of the reasons it is so popular is because of the robust number of plugins that allow webmasters to quickly and easily optimize and customize websites to perform any function or task.
There are literally thousands of WordPress plugins. Some are very universal in nature, while others provide solutions for very specific use cases.
Here are some links to articles that outline and review essential plugins for everything from cacheing to security.