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Links to Add-ons for Microsoft Flight Simulator and Lockheed Martin Prepar3D
Utilities
Accu-Feel by A2A Simulations Enhances sound and some flying effects. Adapts for all aircraft - default and third-party Adds dynamic wind, wheel, rumble and cabin sounds, along with stall buffeting and sounds. For FSX and P3D.
AirWrench (Mudpond) AirWrench is an automated editor designed to create and tune-up flight dynamics files for fixed wing aircraft in FS9, P3D and CFS. 
EditVoicePack Allows you to enhance the ATC speech engine included in FSX & P3D. ATC will be able to use standard ICAO phrases, speak faster, know more call-signs, aircraft types and airport names. Low cost payware.

FlightSim Tools

AI Optimizer is a tool for adjusting levels of detail of FSX & FS9 aircraft. Texture Doctor, a tool for converting FS textures. Addon Converter X, will make add-ons compatible. Airport Wizard. Contrails Pro for FS9 and FSX.
FS Flight
Keeper
FS Flight Keeper combines six programs in one: logbook, aircraft black box, flight weather planning tool, flight information system (ACARS), GPWS and cockpit sounds enhancement. Check out the screenshots.
FS-FlightControl FS-FlightControl is a touch optimized Windows app instructor station for P3D & FSX. Android and iOS apps also. Easy positioning at any approach or location, weather control, fuel load manager, moving map and more.
FSFlying
School
Producers of FS FlyingSchool, an award-winning virtual instructor add-on for MSFS, FSX and P3D. It provides a team of talking instructors that ensure your simulated flights are as real as possible.
FS Real Time Payware (tiered pricing) designed to keep FSX and P3D UTC (Zulu/GMT) time and date in sync with the real world. You can have it maintain either the current real time, or an offset (+/-). Can be used with Time Zone Fixer.
FSWidgets Airports, SlewAssist and Electronic Flight Bag (EFB) for MSFS20, FS9, FSX and X-Plane. Also a range of Android and iphone & iPad apps.
FSXControls FSXControls is for owners of advanced controller system with a lot of programmable buttons (home cockpits, CHProducts' MFC panel etc). Also efficiently and easily hides aircraft variants. Last updated 2014
Instructor Station A useful, interesting interface for FS9 and FSX. Provides loads of settings for a quick launch of the sim in a range of customizable situations e.g. push back, approaches, failures, weather conditions. See video.
Jacky Brouze Some excellent FSX freeware utilities by Jacky Brouze. Jacky is very protective of his JAB creations because in the past his work was badly pirated. Login required. French language.
Lorby Useful utilities from Lorby - Addon Organizer, Scenery Exporter, Thumbnail Report Generator, Mission/Senario Report Generator, Time Machine, Payload Manager, Mooring (floatplane anchor), Aircraft Selector and Scratchpad.  
mixMugz
(@ GitHub)
A collection of small, useful mod's for the MSFS2020 UI by mixMugz (Maksim Brykov). Includes missing device images, hover tooltip options, toggle InGameHud, resize default nameplates with smaller ones, and other mods.
P&J Dowson's Software The latest versions of the Dowson's essential FSUIPC and other utilities including WideFS and AdvDisp. Check out the separate support forums for announcements & support. FSUIPC7 for MSFS2020 released.
Pushback Helper Pushback Helper is an application to make MSFS push back operations easier. Use it to dis/connect the jetway, call fuel truck and move and steer tug during pushback.
RWY 29 Home of Package Manager by Johan Peeters that enables users to easily and quickly mange their Community content. Drag and drop your downloaded files and add them to your Flight Simulator with the click of a button.
SIMstarter NG SIMstarter NG is the payware version of the free SIMstarter. Rewritten for FSX and P3D. You can create many custom launch profiles and combine a profile with any saved flight. Many other functions.
SimElite Solutions Time Zone Fixer is an useful fix for flight simulator software and together with FS Real Time add-on will allow you to fly in FSX, P3D and MSFS on accurate schedules same as the real world.
Tweak FS A series of separate, useful,programs to easily tweak FS9 and FSX flight plans, frame rates, flight characteristics and an aircraft's weight. Config Studio is a "Swiss Army Knife" for your Prepar3D config files.
WidevieW & FS Earth Home of utilities by Luciano Napolitano. Payware WidevieW can be used to create virtual cockpits with multiple monitor support, using networked PCs. FS Earth acts as an interface with Google Earth.