X-Plane Remote Access (XPLRA) plugin

The X-Plane Remote Access plugin allows external applications to access some of the internal state of X-Plane. The plugin has client libraries for C/C++ and Python, but the protocol is also documented, so you can write your own client even from scratch. Currently it is not so "remote", because the external program has to run on the same computer as X-Plane, but it could be easily extended to allow for communication via TCP as well.

The plugin and the client libraries can be downloaded here, and the developer documentation can be read [doxygen:xplra/index.html here] or here.