It is little secret that Joomla! has a thriving 3rd party developer community.  Any function you can think of is probably already available in one of the many 3rd party extensions. It is sometimes difficult to wade through all of the choices that are out there to determine what you need, so I have put together a short list of components that I feel are must-have components. So, without further adieu, here is my list:

Free/Open Source Components 

• JoomlaXplorer - If you do not have this component yet, get it! JoomlaXplorer is a component that enables a site administrator to manage all files within a Joomla! installation. You can use it to upload, delete, copy, move, and chmod (change permissions to) any file that the web server gives you permission to modify. I use to to customize modules and components after I have installed them, customize templates, and upload images. It makes so many functions available right from the administrator interface, you will wonder how you ever lived without it.  Make this the first thing you install on your Joomla! site.
• Community Builder - This is the best user profile component available for Joomla!, and it's absolutely free!  You can use this component to create something as complex as your own version of the popular social networking site, MySpace.com, or something as simple as an extended user profile for gathering information about your users.
• VirtueMart - VirtueMart offers a complete e-commerce solution for your Joomla! website. It would take too long to list all of the things this component can do, but suffice it to say that it is nothing short of amazing. If you plan on running a full-featured online store, this is what you need. Oh, and it's free!
• Forms Generators
  • FacileForms - In my opinion, FacileForms is the most robust, versatile forms generator component available for Joomla!.  You can create entire applications (and some have) using this component.  The only drawback is the steep learning curve.
  • MosForms - MosForms is not as powerful as FacileForms, and it is technically still in beta.  However, I find it a lot easier to use and configure, and it is worth testing to see if it will work for your website.  I've used it in another project, and it works well for me there.