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Links to Add-ons for Microsoft Flight Simulator and Lockheed Martin Prepar3D
Flight Sim 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. Separate versions for FSX and P3D.
AxisAndOhs AxisAndOhs is an external 'stand-alone' application that replaces the controller (yokes, joysticks etc.) logic in FSX, P3D and MSFS.  Supports native Windows USB Game Controller devices, MIDI devices and Saitek panels and FIPs. Steamdeck profiles and other downloads available.
EditVoicePack Allows you to enhance the ATC speech engine included in FSX & P3D. ATC will be able to use standard ICAO phrases, speak faster, and 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 is a tool for converting FS textures. Addon Converter X will make add-ons compatible. Airport Wizard. Contrails Pro for FS9 and FSX.
Flow Pro Flow brings a highly intuitive and easy-to-navigate in-game interaction wheel into MSFS. It can fully replace the sim's default toolbar with a customisable menu wheel. Arrange the wheel's panels to your needs or have them aircraft-specific.  Xbox controller optimised. Widgets on flightsim.to...
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 sound enhancement. Only for FS2004 or FSX.
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, etc.
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 who ensure your simulated flights are as real as possible.
FSRealistic FSRealistic brings a new world of immersion into MSFS with a growing list of 28 sound & airframe vibration effects to experience. Includes turbulence, touchdown, engine vibrations, stall buffet and many more. At Flightsim.to...
FSWidgets Home of QuickPlan that makes it easy to generate flight plan files, and GMapHD that gives you a great moving map for your desktop flight simulator. For MSFS20, FS9, FSX and X-Plane. Also, a couple of  Android and iPhone & iPad apps.
FSXControls FSXControls is for owners of advanced controller systems with many 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 various customisable situations, e.g. pushback, approaches, failures and weather conditions. See video.
Lorby Useful utilities from Lorby - Addon Organizer, Scenery Exporter, Thumbnail Report Generator, Mission/Scenario Report Generator, Time Machine, Payload Manager, Mooring (floatplane anchor), Aircraft Selector and Scratchpad.  
mixMugz
(@ GitHub)
A collection of small, useful mods for the MSFS UI by mixMugz (Maksim Brykov). Includes missing device images, hover tooltip options, toggle InGameHud, resize default nameplates with smaller ones, and other mods.
MSFSAddons Linker
(@ Flightsim.to)
A little addon manager. It lets you organise your add-ons freely in folders and subfolders. It lets you add, update or remove add-ons while creating and/or automatically updating links in the community folder. Activate/deactivate groups of addons with a single click and save presets of groups of addons.
Package Manager
(@Flightsim.to)
Package Manager by Johan Peeters enables users to manage their MSFS community content easily and quickly. Drag and drop your downloaded files and add them to your Flight Simulator with the click of a button.
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 that makes MSFS pushback operations easier. Use it to dis/connect the jetway, call the fuel truck and move and steer the tug during pushback.
SimElite Solutions Time Zone Fixer is a helpful 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, the same as the real world.
Tweak FS A series of separate 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.