There are several developers that create amazing unique add-ons for flight simulators. Today, I would like to highlight the X-Plane add-ons created by FSInventions, who have been flight sim developers for over 15 years! I first started using their software with FSX in 2005, and have continued to use their top quality work as I've moved simulators, first to P3D and then more recently to X-Plane 11. They provide fast support and release many free updates throughout the year. Their software is available from their own website, but also at the X-Plane.org Store (links below). I highly recommend them and hope you also benefit from their unique add-ons.
If you try their add-ons, please let me know what you think of them. Feedback is one of the best parts of running this site.