The guys over at JoomlaShack started a voting last month to find out what the community thinks is the best Joomla! extension. So they asked "What are the top ten Joomla! extensions" and 2.591 people voted.
And now guess what: The "grand prize" winner is: VIRTUEMART!! JoomlaShack will donate $200 to the VirtueMart Project, which can be used for the extension development.
I was really surprised to find VirtueMart at #1, because there so many good Joomla! Extensions. This "eco-system" of Joomla! Extensions is so important for all of us and makes the joomla! community grow each day. Basically you can use Joomla! for anything you want - just create an extension or a plugin ;-) A plugin system for VirtueMart is planned and probably VirtueMart 1.2 will bring it, so you can easily extend VirtueMart as you do with Joomla!.
For some more information about good Joomla! Extensions I can recommend the article 50 Top Joomla! Extensions by "Water&Stone".