The Official WordPress Directory lists thousands of fantastic plugins which optimise WordPress and enhance it's functionality. Some developers update their plugins regularly, either to add more features or to make sure their plugin is compatible with the latest version of WordPress. Understandably, many developers just don't have the time to update their plugins or devote all their time to it due to other commitments (work, family, other projects etc).
Premium WordPress plugins are usually better supported as the developers are getting paid for their work. In general, this allows plugins to be more in depth and add more functionality to WordPress. In this page you will find a list of marketplaces where you can purchase premium plugins. I encourage you to check out my list of WordPress plugin shops as well.