Taken from this Reddit thread most credit goes to chief_187 for accumulating the links.

Weather/Clouds:

Clouds HD Extending the look of the standard clouds. Making the skies more happy.

NOAA Plugin Injecting 8 layered wind data into x-planes weather engine. Simply one of the best plugins for x-plane out there.

Scenery Objects:

World 2 X-Plane World2XPlane will add forests, regional houses and landmarks using a list of highly configurable rules, based on open street map data.

High Performance Textures for W2XP: America

High Performance Textures for W2XP: Europe

High Performance Library

OSM Pro Series Like w2Xplane but even more precise OSM data.

Treelines and Farms Adding tree lines and farms in your scenery. Making the look and feel of the scenery more “organic”.

Enhanced Runways HD Better looking runways… yay!

Scenery orthophotos:

Ortho4XP This uses Google maps / Bing / USGS and many more for generating high quality photo tiles. Highly recommended!

Scenery Zones LowRes photo tiles for Europe.

IT Zones Good photo tiles for just Italy.

UHD Spain HD Photo tiles for Spain and its islands. This is how your photo tiles should look like. Really well done.

Hawaii Photo Scenery Beautiful textures & mesh for Hawaii

Scenery Meshes:

UHD Mesh 30m DEM resolution data.

HD Mesh 90m DEM resolution data.

Visual Enhancement:

Flight with LUA Lua 5.1 script engine for X-Plane. You need it for RTH.

RealTerraHaze Adding atmospheric scattering, Raleigh and a lot more. Really a must have!

European City Lights Adds more street lights in cities, they will look brighter. Deletes streetlights at highways.

BluFX Lets you change saturation, contrast, etc. Includes a frame limiter, too.

Airports:

Airport db

X-Plane.org

MisterX6 Scenery

TDG Scenery

Aircraft:

Download section of the Org.

B 737-800 Great jetliner + 3D cockpit

LT-401 Small turboprop + 3D cockpit

Fokker F27-600 Turboprop airliner + 3D cockpit.

Douglas DC-9 Great jetliner + 3D cockpit

Eclipse 550 Light business jet + 3D cockpit

Mil Mi-2 Former payware helicopter now free! + 3D cockpit

Verticopter VTOL aircraft, fun to fly. + 3D cockpit

Sikorsky S-76 Nice helicopter, flies well but not very deep systems wise + 3D cockpit

Kalinin K-7 Russian monstrosity that allegedly can fly + 3D cockpit

Aero Boero AB-115 Former payware bush plane +3D cockpit

Shavrov Sh-2 Small amphibious aircraft +3D cockpit

Mitsubishi A6M Zero Beautifully crafted WWII airplane + 3D cockpit

Jetpack: JETPACK.

Yak 55: Aerobatic plane which used to be payware, now free!

Tools:

Plan-G: Donationware for flight planning, is able to connect to XP via XPUIPC and lets you program the AP, change radio settings, etc. POWERFUL TOOL

EFASS: It is a swissknife for X-Plane. It can do a lot but nothing really good. I would suggest, you should take a look at the demo as it is freemium.

X-Camera: X-Camera is an enhanced camera system for X-Plane that lets you define multiple view categories and multiple views within those categories that are associated with specific aircraft.

XHSI: XHSI (eXternal High-resolution Simulator Instruments) is an add-on for the X-Plane flight simulator. It displays the PFD, ND, EICAS, Flaps and Gear indicator, Clock/Chronometer, etc… in separate windows. In general, the instruments can be displayed in B737-NG or A320 style.

Landing Speed: Landing Speed plugin grades quality of landing. It shows vertical speed at the moment of touch down in feet per minute and meters per second as well as textual grade of landing quality.

EZ Pushback: EZPushback is a lightweight pushback utility designed for X-Plane 10.20 or later. It simply moves the aircraft backward or forward. It can be controlled by menu, keyboard, joystick, or from other plug-in.

Ground Services: Provides ground services with object animations using OpenSceneryX objects. A nice plugin to have some movement around your plane, and do your push-back and refueling

xOrganizer: A great tool for X-Plane! Manipulates scripts, lets you review all the addon scenery on a map, mesh coverage etc. Pretty exceptional.

X-FMC: Universal FMC. This plugin helps you following IFR flightplans on X-Plane and interacts with X-Plane default autopilot.

SkyVector: Excellent flight planing site.

X-Assign: Lets you set up your controls PER AIRCRAFT. Comes very handy when you switch from planes to helicopters regularly.

Airport Navigator: This plugin provides an overview map of the airport you’re at.

LiveATC Lets you listen to real ATC radio traffic, excellent background noise.

Apps:

FltPlan Go for Android & iOS: Description from the App store: „The free FltPlan Go app is an indispensable companion to the FltPlan website and incorporates essential features and tools for in-flight and offline use.“ North-America only.