iJoomla.com - Offers a mixture of free and paid extensions for your site.
Joomla-Addons.org - A large selection of free addons for Joomla.
JoomlaEquipment.com - Professionally developed extensions for Joomla.
JoomlaHacks.com - A large collection of free modules and templates for you to use.
JoomlaShowroom.com - Designing extensions for Joomla-based business sites.
JoomlaSpan.com - A selection of paid extensions, and a subscription based monitoring service.
HowToJoomla.net - A blog dedicated to everything Joomla and offers lots of tricks for advanced features.
Joomla.org - The main home for the open source project with an archive of 2,200 extensions for your install, a forum, a developer area and more.
Joomla-Developer-Resources.com - Tutorials for getting started with your own Joomla Development.
JoomlaBased.com - Submit your site and get reviews and comments.
JoomlaCode.org - A resource site for those looking to develop projects based on the Joomla code.
JoomlaDirectory.co.uk - A UK based directory of all the Joomla resources out there.
JoomLancers.com - A site for people needing special work down on Joomla to find freelancers looking for work.
JoomlaPoweredSites.com - A site for you to submit your Joomla powered site to and receive comments and reviews from users.
JoomlaSpots.com - Submit your Joomla based site to this directory to get yourself more exposure.
JoomlaTribune.com - Bringing together news and resources from all over the Joomla community.
JoomlaTutorials.com - A large selection of tutorials from installing to adding extensions.
JoomlaZone.net - A site with the aim of bringing all Joomla sites together in one easily accessible reference directory.
CompassDesigns.net - Professional template designers with ready made solutions. Also offers free tutorials.
Gavick.com - A European based designer of professional templates with a small selection of free ones.
IbolDesign.com - A collection of free Joomla templates for your use.
Joomla-Themes.co.uk - News and reviews on all the latest themes out in the Joomla world.
Joomlart.com - A template club where you get 40 templates right away, and 2 per month for the next year. Does have a few free templates, and a forum.
JoomlaJet.com - A Joomla theme club with 50 themes already in their backstock.
JoomlaJunkie.com - A professional template design company that likes to focus on color. Offers a small selection of free templates.
JoomlaShack.com - Offers up a small selection of free templates and more you can buy. Also has tutorials and a forum.
PixelParadise.com - Offers over 60 pay for template designs.
RocketTheme.com - Another template club that offers you 45 templates at sign up and 1 per month for a year. Offers a small selection of free templates.
Shape5.com - Offers a mixture of paid for and free templates & modules.
SiteGround Joomla Templates - Well known web host offering over 50 free Joomla templates.
YouJoomla.com - Joomla templates for business style sites. (via)