Dronecode (https://www.dronecode.org/ ) : An open-source project managed by the Linux Foundation.
DIY Drones (http://diydrones.com/ ): A community site with resources for building drones; hardware and software areas are both covered.DroneKit (http://dronekit.io/ ): Another open-source App. development platform, from 3D Robotics.
DJI SDK (https://developer.dji.com/ ): Suitable for drone App. Development; there is a mobile SDK, an onboard SDK, a guidance SDK, and SDKs for Android and iOS.
Parrot AR SDK (http://developer.parrot.com/ ): Covers the basics of drone development; including touch and tilt flight controls, live video streaming, video recording, and photo shooting. (Microsoft has used this, together with Kinect.)