Joomla Extensions Store
On Joomla Extension Store site you can download plugins and Joomla templates free and paid. There are many professional paid extensions and many other free extensions that do an excellent job.
JSitemap has become a first class extension on the JED (Joomla Extensions Directory) and it's an extension to generate the sitemap of a Joomla website without external plugins and components. You can also find JChatSocial that allows you to easily add a realtime chat on your Joomla site and many other Joomla components and Joomla templates.