FlightGear

FlightGear is an open-source equivalent to X-Plane. It is truly cross-platform, running on Windows, Linux, Mac OS X, FreeBSD, Solaris, and IRIX. Although it is not as refined as Microsoft Flight Simulator or X-Plane, I would defitely consider FlightGear as a serious tool for simulation research. Unlike the commercial products, the researcher retains complete and final control over the software. The researcher is also able to understand every aspect of the underlying algorithms, and modify them if necessary. Naturally, this requires a fair amount of programming knowledge. A scenery editing tool, FlightGear Scenery Designer, is also available but I have not tested it because the installation is complex and it has not been packaged for Gentoo Linux.