After several years of using Joomla, it has become less and less a complete content management system, and more and more a framework to support a useful collection of extensions. With 8,500+ extensions, there are some extensions that I have come to love and rely on to get the job done well. The extensions below are well supported, not buggy and extremely flexible.

1. JCE Editor: http://extensions.joomla.org/extensions/edition/editors/88
   There simply isn't a better content editor out there. It most definitely blows TinyMCE out of the water. Creating links is so much easier, inserting photos is easier, and formatting your text is easier. It makes creating content inside of Joomla much less of a chore, and creates a more familiar framework for end users who may be used to Microsoft Office Products. I also recommend purchasing a JCE Subscription and downloading the Advanced Image Manager and Media Manager. The bulk image resizing feature in the Advanced Image Manager is, by itself, worth the cost of the subscription.
   
   
2. NoNumber Extension Manager: http://extensions.joomla.org/extensions/core-enhancements/installers/17071
   This extension actually comes packed with over 20 different extensions. I haven't had a chance to test all of them out, but my absolute favorites are the Better Preview plugin and the Advanced Module Manager. Better Preview places a preview button on every article and menu item page, adding a sorely needed feature to the Joomla Core. Advanced Module Manager expands the abilities of the core module manager by giving users many more rules they can use to assign modules to a specific page. Users can assign modules to articles, categories, user groups or can even assign modules based on browser or operating system, among other options. Honorable mentions with the NoNumber Extension Manager include the Add to Menu plugin, the Advanced Template Manager, Email Protector, Sliders and Sourcerer. The other extensions available through NoNumber are probably excellent too... I just haven't had the opportunity to personally test them.
   
   
3. Chronoforms: http://extensions.joomla.org/extensions/contacts-and-feedback/forms/1508
   There are a number of form building components available for Joomla. Indeed, Joomla comes with a very basic form builder in the "Contacts" component. Chronoforms is my very favorite for two reasons. Firstly, it can be used to easily set up a basic form with a drag and drop editor and a decently simple interface. Secondly, if you want to dig into the nuts and bolts, and create a database driven application, that capability is also there. Chronoforms is not perfect in my opinion however. There are a few little irritations that come with the application. When one wants to create a link to a form, they have to manually type in the name of the form, rather than searching for it with a browser or an ajax driven field. When creating fields, even in the simple mode, each field's options can be a little overwhelming for beginners (label vs. field name vs. field id vs. field value). But for a builder like myself, these problems aren't enough to dissuade me from recommending this extension.
   
   
4. DJ Image Slider: http://extensions.joomla.org/extensions/photos-a-images/slideshow/image-mootools-slideshow/13119
   If you search for "Image Slider" in the Joomla Extensions Directory, you'll come up with dozens of possibilities. I've used quite a few of those options, but I have always come back to DJ Image Slider for it's ease of use and versatility. This extension is a combination of component and module. It can be set up to display images from a folder, or you can create individual slides in the DJ ImageSlider component. Individual slides can be linked to articles, menu items or URLs. The code driving DJ Image Slider is very clean and to the point. It's easy enough to edit it to fit your needs. As a designer, this extension is one of the very best Image Sliders out there.
   
   
5. Podcast Manager: http://extensions.joomla.org/extensions/multimedia/streaming-a-broadcasting/audio-broadcasting/16500
   I build websites for a lot of churches. Most of those churches want to allow visitors and church members to easily download MP3 recordings of their sermons. The Podcast Manager component is the very best option here for its ease of use, its easy integration with iTunes and the support of its developer, Michael Babker.
   
   
6. SOBIPro: http://extensions.joomla.org/extensions/directory-a-documentation/directory/16649
   This custom directory extension isn't necessarily the easiest to use out of the gate, but it's abilities are pretty limitless, especially if you are willing to work with it and understand how it works. SOBIPro allows users to create forms that can be filled out to create content on the front end. It is similar to the very popular K2 component, but takes things to a more powerful, industrial scale. I have personally used it for church directories, but it could be used for any kind of front end, template driven content.
   
   
7. RemoteImage: http://extensions.joomla.org/extensions/photos-a-images/images/image-management/23952
   One of the biggest nagging issues with Joomla since the beginning has been its Media Manager. It has gone relatively unchanged for the past 7 years. To the uninitiated, the Media Manager allows end users to upload photos, create folders and preview photos. It lacks any other features (renaming files, renaming directories, moving files and directories, editing photos, drag and drop ability). The RemoteImage application is exactly what the Media Manager should be. It not only offers an alternative to the core media manager, but can actually replace it in all instances on the website. This includes the "Images and Links" tab in the article editor and the "Media Manager" link in the content area. The only negative I have with RemoteImage is it's reliance on somewhat ambiguous tiny icons for different file operations. This issue isn't a deal breaker however, and is easy to put up with compared to the extremely limited abilities of the core media manager.
   
   
8. Simple Image Gallery: http://extensions.joomla.org/extensions/photos-a-images/galleries/content-photo-gallery/1468
   To whip up a gallery in seconds, go with Simple Image Gallery. Simply upload your photos to a folder, click the gallery insert button in an article, and your done. If you combine Simple Image Gallery with the JCE Advanced Image Manager, you've got a great tool. JCE's image manager will resize your photos in bulk, and the image gallery will display them nicely.
   
   
9. Attachments: http://extensions.joomla.org/extensions/directory-a-documentation/downloads/3115
   There are some really good, comprehensive document managers available in the JED, but if all you need is a simple way to allow visitors to download one or two documents, it's hard to beat the Attachments component and plugin. It's use is dead simple for end users. Open an article, click the attachments button, select a file, clip upload and save. It works in a very similar fashion to attachments in email. It will create a download link section at the bottom of the article (or the top, if you choose). If you need something more powerful, I'd recommend DocMan.
   
   
10. SP Upgrade: http://extensions.joomla.org/extensions/migration-a-conversion/joomla-migration/15609
    This is hands down the very best extension for migrating your Joomla content from an older version to a newer version. Simple install a new Joomla site, install SP Upgrade, connect it to the old site's database, and import your content. Personally, I've found I get the best results when I don't try to import everything all at once. I will usually break up categories, articles, menus and menu items and do those one at a time (one group at a time).
    

Honorable Mentions: Other extensions I love using include JEvents, K2, Disqus Comments, FileMan and ACL Manager.

Do you know of any extensions I missed? Post those in the comments below.