These who ever worked with Content Management Systems will know about Joomla! CMS. Personally, what I like about Joomla! is availability of the vast number of extensions and templates and many of them are available for free. One of the popular sites where you can find Joomla! extensions is extensions.joomla.org. There are a lots of extensions for all sort of tasks. Here I am listing the ones I am using in my projects, which are tried and tested.
joomla eXtplorer is a File- and FTP - Manager. It allows you to edit files, delete, copy, rename, archive and unpack files/directories directly on your server. You can - Browse Directories & Files - Edit, Copy, Move and Delete files - Search, Upload and Downloading files - Create new files and directories - Change file permissions (chmod) and much more. I am mainly use this extension for changing file permissions and editing configuration files.
Linkr - The Article Linker and Link Creator
This extension lets you quickly link to articles in your Joomla! site when you are creating content. You can either search for your article with the search box, or find it by selecting the section and category. Finally, the link class, title, relation and url are completely customizable, making Linkr the perfect linking extension for Joomla! I like the nice user interface allowing easy article categories browsing and article preview.
Xmap is a Site map generator component for Joomla, it's based on the popular Joomap component. Xmap allows you to create a map of your site using the structure of your menus. In the Preferences Page under Options there are links to html sitemap page and xml sitemap document which can be submitted to Google webmaster tools.
sh404sef is one of the commercial extensions I would highly recommend if you are serious about SEO (Search Engine Optimisation). This extension make it very easy to control page titles and meta tags and automatically converts Joomla links to search engine and human friendly links. By itself it has a lot of extensions that optimise this component for other Joomla systems like VirtueMart, Xmap, Book or Media libraries. Check these extensions at sh404SEF extensions.
I will be happy for your comments about what extension you use and would recommend to others.