Gliders - current recommended gliders for X-Plane, including the ASK21_B21 and ASG29_B21.
X-Plane Gliders Facebook group (requires Facebook account). Ably supported by Nodd, promotes multi-player soaring in X-Plane.
The new Gliding in X-Plane Forums created in June 2019. With a bit of luck this will build collaboration for soaring in X-Plane.
The basic thread on X-Plane Forums
Library of useful manuals on X-Plane usage and developemnt
The official X-Plane developers website includes manuals and tutorials.
X-Plane Plugin SDK reference data, for when you reach the scary edges of your Lua development.