Recomended Joomla extensions
There are few extensions that I install by default with every new Joomla installation. They are all free Joomla addons.
The first one is JCE editor. JCE editor is extended version of integrated TinyMCE editor. It offers more functionalities then default editor. Since a majority of work is adding content, it makes sense that the component responsible for this job is as comfortable as possible.
A component for making backups of Joomla site. This is a must have extension for security reasons. It is very simple to reinstall Joomla using this tool. You just have to download a backup file to root folder on your web server along with included kickstart utility. Once uploaded one has to start kiskstart utility and follow instructions. There is also a desktop utility named Native tool to facilitate a manual backup. It is very useful when it comes to maintenance of more then just few sites.
Joomla is one of the most popular open source web page design frameworks available at the moment. With increasing popularity of Joomla CMS, grew also the interest of hackers to exploit various security holes of Joomla. To protect your site against this type of malware, see RSFirewall! extension which is compatible with Joomla 1.5, 2.5 and 3.0.
Joomla update manager
Due to security leaks a new Joomla patch is available every while. To install it manually is quite a tedious work. This Joomla addon come as a relief. It is 1, 2, 3 clicks easy upgrade procedure. Basically you just have to confirm steps and voila: the newest version of Joomla is installed in less than a minute. This should be a core component. Hope with Joomla version 1.6 this will be the case.
This is a very simple but useful plugin. It makes a thumbnail presentation of picture you download. You just have to change one dimension of picture (horizontal or vertical) in Joomla text editor. Plugin will automatically expand a picture to its full size, when you click on thumbnail.
Joomla Extensions directory
You can visit presentation, comment and download page for mayority of free and commercial extensions available for Joomla at Joomla extensions Directory .