These are the list of the joomla extensions that i use often for the client websites. Joomla extensions that are useful, There may be many useful extensions than this, but these are the modules that i have used frequently for other projects.

1. Ari Image Slider
   Ari Image Slider module which provides possibility to create awesome image slideshow based on images from specific folder
   
2. Maxi Menu CK
   Maxi Menu CK is a RESPONSIVE Megamenu dropdown with multicolumns and multirows.
3. AcyMailing
   AcyMailing is a reliable Newsletter and email marketing extension for Joomla.
   AcyMailing enables you to efficiently manage an unlimited number of subscribers, organize them into mailing lists, send personalized newsletters
4. AllVideos
   You can use the plugin to easily embed media hosted on popular services like YouTube, Vimeo, Dailymotion, SoundCloud (and many more) inside your Joomla! articles.
   Additionally, it allows you to playback almost any video/audio file format hosted on your server or even a remote server.