There are numerous free extensions of Joomla that can be used commercially. However, only a nominal number deserve being top of the list. Some top of the line GPL extensions have been featured in this list below:
JoomlaXplorerThis is a complete file manager working within Joomla.
Plus points: Features list, quality, ease of usage, integration to the Joomla.
sh404SEFGreat extension for joomla seo and meta control. This SEO component supports popular extensions and has relative ease of usage.
Plus points: Ease of setup, supporting other extensions, multiple options.
KunenaKunena is one of the best forum component for Joomla. Total integration with Joomla.
Plus points: Easy setup and maintenance, loads of features, speedy, neat appearance
Joom!FishThis is a perfect answer to problems faced in accessing a multi-lingual website. The core extension comes without any charge and enables you to translate any contents within Joomla.
Plus points: free tutorials, open forum, loads of features
Minuses: All extensions do not work properly, support not up to the mark.
Community BuilderBuilt for social networking, with multiple features untouched till date by anything else.
Plus points: Great user management control, multi-optioned usage.
Minuses: Bit complex for novice users of Joomla, minor compatibility issues exist.
MiniFrontPageHelps displaying the latest additions to users content in module position. Content can be displayed with automatically generated image and a customized link for reading more.
Plus points: Custom content is easily integrated into module position, multiple layout
options, loads of features.
JoomlaStatsGreat extension for Statistical trackings. It is very useful for tracking referrals and visitors. http://www.joomlastats.org/
Plus points: Great working, easy to install and use, superb add-on module provided.
Akeeba backupThis is the open source component for backup for Joomla. Its operational model is simple: creating a backup site that is restorable anytime within the Joomla server.
Plus points: Ease of automating database in Joomla, convenient to use, file exclusion
VirtuemartThis free extension makes user really understand the greatness of Joomla. Provides Joomla-integrated solution to people wishing to sell things. A different style of working, superb availability of documentation and a great file extension are the key features.
Plus points: Free shopping-cart solution from Joomla, availability of great
Minuses: Limited support from developers have caused unresolved problems for
Extended MenuAllows displaying of dropdown menus, with great power and functions. Great help to developers as it provides a compact base to build a customised menu system.
Plus points: Great Power, options to customise, versatile.