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Links to Add-ons for Microsoft Flight Simulator and Lockheed Martin Prepar3D
Artificial Intelligence (AI) Air & Sea Traffic
Aerosoft Tool Simple Traffic Change the AI traffic liveries in MSFS from default fictitious airlines to real airlines. Over 160 airlines and over 1400 traffic routes for airports included (more added with every update). See webpage for limitations. AI Flight Planer
AI Flight Planner AI Flight Planner from Stuff4FS is a fully-integrated, TTools format-compatible AI flight planning tool for Microsoft Flight Simulator. Aircraft may be assigned directly from FS9, FSX and P3D aircraft folders.  Alternative download site... AI Flight Planer
AI Tracker X A useful payware AI tool for FSX or P3D. It offers multiple modes, easy formation flights, recording & playback. Follow AI using an invisible camera etc.  Lorby SI logo
Airport AI Enhancer Gunnar Daeling's Airport AI Enhancer (Freeware) helps you to fix the problem of empty aprons at your favorite FS9, FSX and P3D. Also has other utilities. Airport AI Enhancer
Alpa India Group (AIG) AIG are a group of Flight Simulator fans who want to put the fun back into AI. Aircraft models including Antonov, Boeing, McDonnell Douglas, CRJ etc, repaints, flight plans and AFCADs. Forum. Also have a Facebook page. AlpaIndia
Doug's Flights of Fancy AI repaints and some interesting flight plans and AFCADs for statics at various location, civil and military, including the famous airliner boneyard at Kingman Airfield, Arizona. For FS9. Also videos and links. Doug's Flights of Fancy
FAIB
(FSX AI Bureau)
FSX AI Bureau (FAIB) for simmers who want native AI models for FS9 and FSX. Boeing and Airbus models in a large variety of airline liveries.
Flying Carpet Hub A huge selection of detailed repaints for FSX, P3D & MSFS AI aircraft. Includes liveries of Hong Kong Airlines, Royal Air Maroc, FedEx and many others. Also AI flight plans and call signs and links. Contains a painting tutorial. Flying Carpet Hub
FSTL FSLTL is a free standalone real-time online traffic overhaul and VATSIM model-matching solution for MSFS. Populates your skies with live aircraft fully interreacting with default MSFS ATC. Easy to setup and use via installer tool, no complicated configuration. VATSIM Ready for online use.
FS Traffic from Just Flight FS Traffic features a fleet of brand-new aircraft models, designed to exploit the enhanced visuals of MSFS, with excellent FPS performance. The aircraft include high definition livery textures, animations, night lighting & more. Adds tens of thousands of real-life commercial flight plans giving full worldwide coverage of commercial airline traffic. 
FS Reborn AI Lights Reborn Pro edition fixes the missing AI traffic nav lights in P3D V4 & V5  (AI strobe lights invisible at distance, nav light orbs, ugly lights, etc.). Also enables some special effects for AI Traffic. 
Global AI Ship Traffic Thread on AvSim about the freeware Global AI Ship Traffic project by Henrik Nielsen & team. The thread is long, so make sure you find the latest news & support info. A number of files are needed. A similar thread on FlightSim.Com Global AI Ship Traffic
HTAI Models Henry Tomkiewicz's AI models. A large range of  civilian and military AI models. Also the payware Cessna Project containing models of over 50 single-engine props from various eras of Cessna's production history. Mainly for FS9.
Juergen's paint hangar A large collection of detailed repaints for FSX & P3D AI aircraft including rae and unuslal liveries. Includes collections from Marco and Nils.
MAIW MAIW (Military AI Works) produce packages of military AI aircraft. Aircraft and Sets cover many countries of the world and offer a huge range of aircraft from fighters, to heavy lift, to helicopters. 
My Traffic X Professional My Traffic X Professional enhances FSX and Prepar3d with realistic, worldwide AI traffic. Includes airliner, general and military traffic, flying millions of weekly flights and an traffic editor.  Version 6.0 released. Forum. My Traffic X Professional
PSXseecon
Traffic
With PSXseeconTraffic (PSXT) you can fly world wide with real live traffic in FSX, P3D & MSFS. Flying in live traffic gives you an awesome immersion. See the manual for additional resources needed. Avsim Forum...
Retro AI Traffic Group Source of vintage flight plans, AFCAD files, and resources. Flight plans are meticulously developed using the finest resources and tools to ensure quality and realism. Web site is down, see forum for screenshots and links to files. Retro AI Traffic Group
Seafront Simulations Overhaul the default global AI leisure, cruise and cargo traffic with high quality, night lit, PBR sailing yachts, catamarans, yachts, fishing, cruise and cargo ships! Also enhanced shipping routes from real-world, global AIS.
RealTraffic RealTraffic consolidates data from several international networks of real airplanes near the position of your simulated aircraft - in realtime, live! It then injects this traffic to your flight sim so you can fly with it, or follow it.
Traffic Global (by Just Flight) Traffic Global for FSX and P3D is the latest incarnation in Just Flight's Traffic series and features thousands of detailed but FPS-efficient commercial airline and general AI aircraft.
Ultimate Traffic II (Forum) Ultimate Traffic 2 from Flight 1 provided FSX users the definitive answer to their AI Traffic needs. See announcements about future development and support status. Updates or new features unlikely. Very limited support. Ultimate Traffic II
United Traffic Team (UTT) Freeware and payware AI models and AI re-paints in many international liveries. Also have AI flight plans and AI helicopters and also a few rare repaints for popular payware flyable aircraft for FS9 and FSX. United Traffic Team (UTT)