Written by Webmaster-tips.net

 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:

sh404SEF

Great 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.

Minuses: Too many options can overwhelm novices.
More info on sh404SEF

Kunena

Kunena 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

Minuses: Allegedly easy for spam bots to attack
Kunena - More Info  

Joom!Fish

This 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.

Joom!Fish - More Info

Community Builder

Built 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.

Community Builder - More Info  

MiniFrontPage

Helps 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.

Minuses: A minor setup is required.
Mini Front Page - More Info

JoomlaStats

Great 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.

Minuses: Page loading gains weight (not easily noticeable)
Joomla Stats - More Info

Akeeba backup

This 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

feature.

Minuses: Software is free, but support is charged.
Akeeba backup - More Info

Virtuemart

This 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

documentation.

Minuses: Limited support from developers have caused unresolved problems for

many.
Virtuemart - More Info

Extended Menu

Allows 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.

Minuses: Advanced parameters of usage, difficulties experienced in setup.
Extended Menu - More Info