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Links to Add-ons for Microsoft Flight Simulator and Lockheed Martin Prepar3D
Flight Planning - Charts - Navigation - Missions
Also, see AppsFlight DecksHardware and Panels pages for avionics
Airport Nav Finder Airport Nav Finder logoAn online, community-maintained database of details of thousands of worldwide airports. Contains category, runway, frequency and weather details, local nav aids and a list of other airports nearby.
Dauntless Software Dauntless SoftwareDauntless sells real-world and simulator aviation products, including SimPlates X - over 30,000 real IFR approach plates for airports worldwide with an integrated worldwide airport/facilities directory. Also, free aircraft checklists.
FSPM
(@ flightsim.to)
It’s an add-on map showing within MSFS with your plane moving in it, without any other external apps needed to run. It does not open with the "V" key and does not overwrite the default map. You can find it in the top menu.
Free AirMap Gemini Software logoFree AirMap from Gemini Software is a Garmin-style moving map standalone program that integrates with FS9, FSX, P3D & MSFS. It shows your aircraft's position and flight plan. Many map types to choose from.
FS Academy Take command of an aircraft, learn instrument and visual flight, handle emergencies and journey across the globe. A real airline captain builds these mission packs for MSFS for maximum authenticity. New! Navigator set. 
FS Aircraft Dynamics & Navdata FS Aircraft Dynamics & Navdata logoHervé Sors designed this freeware material for MSFS, and navaids and Magnetic Variation data updates for FS9, FSX, P3D & MSFS. Also aerodynamics, aircraft data, airfile design data and links.
Flight Simulator Navigation Ruud's blog offers excellent tutorials and YouTube videos teaching you about charts, flight planning, departure (SID) and approach (STAR) procedures, using GPS or radio beacon flying with NDB, VOR, DME and ILS. 
FS Tramp FS Tramp logoSuccessor to FS Navigator - a fully integrated, moving navigational map plug-in for FSX, MSFS & P3D. Can fly entire flight plan route and holdings through autopilot. AI and multiplayer aircraft are shown on the map. Annual payable license required.
FSWidegets QuickPlan and other apps from FSWidgets help generate flight plan files in native MSFS, FSX, P3D, FS2004 or X-Plane formats. Available for ios, Mac and Windows devices
FSXPilot FSXPilot is a generic, universal autopilot, EFIS and flight planner module for MSFS, P3D and FSX that extends the confines of conventional autopilots. It includes a flight planner and a global navigation system. Shareware/donationware for non-commercial use.
EasyFMS EasyFMS imageRunway Advisor, EasyFMS & External EasyFMC software products for FS9, FSX and P3D by Eric Marciano offer advanced navigation systems. Designed for airliners, you can use them with any aircraft. New! MSFS Recorder.
Ideal Flight Ideal Flight logoIt takes the burden out of setting up an FSX flight. One-click creates a completely new flight each time with weather, flight plan, and briefing. It works with any add-on, and its features make it a useful addition to FSX and P3D.
Kip on the Ground Useful, easy-to-follow tutorials on using the autopilot and other navigation instruments in MSFS2020. They include IFR ATC clearance from the cockpit, ILS approaches and Garmin 530/430 GPS flight planning.
Little Navmap Little Navmap logoFree, open-source flight planner, moving map and navigation and airport info app for Windows, Mac or Linux. Supports MSFS, P3D and X-Plane. New MSFS 2020 functions. Reads sim's default and add-on scenery data. Support Forum at Avsim.
Navigraph Navigraph logoNavigraph was founded in Sweden in 2003 and provides flight sim users with up-to-date data and charts via a paid subscription. FMS data (AIRAC cycles) and airport and en route charts are available for desktop or portable devices.
Our Airports Our Airports logoOpen-data details of airports and heliports all over the world. Includes general pilot and passenger information and a direct link to a satellite view of each airport. Useful for general information.
Professional Flight Planner X Professional Flight Planner X logoPFPX is a new planner developed by real-world pilots and flightsimsoft.com. The aim is to support the serious flight simmer to produce complex flight plans.
Plan-G
Plan-G logoPlan-G is a donation-ware VFR flight planning tool for FSX, FS9, P3D, and X-Plane. It features a map interface with topographical detail. Plans are fully editable. You can do the planning outside of FS. VATSIM support and radio stack.
PRO-ATC/SR Pro-ATC X logoPRO-ATC/SR replaces and upgrades ATC in MSFS, FSX & P3D. Pro-ATC/SR provides the appropriate controllers who give you current weather, clearances, taxi instructions, take-off clearances, ascent & descent clearances, and many others. Pro-ATC/SR utilises a virtual co-pilot whenever possible.
RFD
Random Flight Database (RFD) is easy to use and offers various features for random flight generation. It has an extensive database of airlines, airports, and destinations, allowing you to choose one or more VFR and IFR flights according to your criteria. The full version, which includes EFB features, requires a subscription, but the Lite online version is free.
Rolling Cumulus
(at Justflight)
Rolling Cumulus logoWe aim to provide FSX, P3D and MSFS pilots with the best missions that can take you to unique and rare airfields across the globe. The missions usually include several flights, weather themes, custom surroundings etc.
Route Finder Route FinderRoutefinder is an online world-wide aeronautical database. Request a route between two points, and the program will find the shortest path and provide a simple flight plan.
SimBrief SimBriefSimBrief by Navigraph is a virtual flight dispatch centre. Registered users can access an online dispatch system, generating in-depth flight plans for specific aircraft containing weather forecasts, NOTAMs, fuel planning and more.
SimBrief Panel
(@ Flightsim.to)
Toolbar widget for MSFS2020 to display the latest SimBrief flight plan. You can easily download all the most critical data without leaving the simulator. It's useful if you only have one monitor or you're in VR.
SimuFly.com SimuFly.com logoA Delco Carousel IV-an Inertial Navigation System (gauge) for MSFS. All displays, keys and switches work exactly as the real ones. Computes, manages and displays true heading, the distance between waypoints, and more. Forum
SimToolkitPro With support for FSX, P3D and MSFS, and an unrivalled EFB toolset, SimToolkitPro brings your flying experience into one place. Flight planning and dispatching through SimBrief, flight logbook, live map with support for VATSIM and IVAO,  ATC taxi overlays, searchable real-world routes database and more.
Sky Vector.com SkyVector.com logoSkyVector.com is a way to view pan and zoom US aeronautical charts online. FAA Sectional Charts are always current, free, and used for VFR flight planning. Load SkyVector inside MSFS with this free toolbar mod...
TOPCAT Topcat logoTOPCAT (Take-Off and Landing Performance Calculation Tool) is a highly realistic, easy-to-use planning tool. It brings into a single application all of the calculations required to plan for safe and repeatable take-offs and landings.
UK Aeronautical Information NATS logoUK  Aeronautical Information Service. Real-world aeronautical and aviation information from the UK's CAA and NATS. The website includes NOTAMS and various charts that are available for download.
Volanta Volanta by Orbx is an easy-to-use smart flight tracker that integrates all your flight data across all major sims. It includes a logbook, aircraft tracking, fun challenges, and a thriving community. Desktop or app option. Free or Premium option.
World Tour Flights VoxATC logoWorld Tour Flights is community-driven, and everyone is welcome to post MSFS flight plans as PLN or FLT files. Plans are sorted by region, environment, or terrain and can be viewed on a map before download.