AITrack: A Completely Free Headtracking Solution

From the Creator: "AITrack is a 6-Degree of Freedom headtracker designed to work alongside Opentrack for its use in simulators/games. AITrack uses its own tracking pipeline (based on neural networks) to estimate the user's head position with respect to the webcam and then, streams it to Opentrack, which in turn, transmits it to your games."

Supports Digital Combat Simulator, Microsoft Flight Simulator, and many other titles.

ViewTracker: A Webcam-Based Headtracker

From the Developers: "ViewTracker is a gaming utility that turns your webcam into a head tracking controller for games. Simply launch any supported game while ViewTracker is running and begin using the position of your head to change your view in-game."

Supports American Truck Simulator, Arma 3, Digital Combat Simulator, Elite Dangerous, Euro Truck Simulator 2, GRID, Kerbal Space Program, and X-Plane 11.

FreeTrack

FreeTrack is a free software that can take input from headtracking software such as ViewTracker and sends it to your games, such as DCS World and Microsoft Flight Simulator. Depending on your hardware setup, FreeTrack can be used as a standalone headtracking solution (requires LED lights on your head to track properly with a webcam).

OpenTrack

Opentrack is a free, open-source headtracking software that is compatible with numerous games such as Microsoft Flight Simulator, FlightGear and X-Plane 11. It is also compatible with a FreeTrack setup. This software requires a head mount, like FreeTrack, in order to work properly, such as a phone (I don't necessarily recommend this, a 2 or 3-LED mount is better).