Magento is an Open Source Ecommerce platform that provides everything that a shop owner would need to begin selling products online. The Magento Community Edition is the open source edition of Magento which is free to use. The Core Magento development team has worked hard to ensure that Magento is very extensible, making it easy for others to build additional software.
If the standard Magento Community Edition does not provide the exact functionality that you're looking for, then you can extend your installation with Magento Extensions. Third party developers are responsible for building and supporting the individual extensions.
Here's a list of Magento Extension directories where you can find additional functionality for you Magento Installation.
The extensions listed here are not officially supported. This is a directory to help facilitate making it easier for you to connect with the third party developers. This directory contains both free and paid extensions. The free extensions can easily be added to your installation using the Magento Connect Manager that is in your administrative area.
aheadWorks is a third party developer that builds a number of high quality and well known extensions. The support provided by aheadWorks is top-notch and the plugins all work as advertised. This company has built extended functionality for nearly every feature within Magento.
This marketplace offers a few Magento extensions that have been quality assured to a higher standard than most. The marketplace is also made up of extensions provided from third party developers. All extensions within this marketplace are paid extensions. The MageStore Directory provides enterprise level extensions for the Magento Community Edition and the Magento Enterprise Edition. The extensions that you will find here are all built and supported by MageStore. If you're looking for the ability to extend your stores functionality for use with fulfillment and inventory management, then this would be the place.