Joomla is one of the "big 3" opensource Content Management Systems (CMS). You could always download it at Joomla.org, but there's a new, free way to try Joomla. You can now go to Joomla.com and build a free website. It's a service offered in conjunction with a commercial hosting service, SiteGround, so of course they make their money by upselling you to their paid plans.
The service is somewhat new (end of January 2015 launch) so there's still some rough edges and a limited number of templates. You can't install your own extensions, much the same as the free Wordpress offering.
See our WebSiteOwnersGuideBook demo site there.