I've been experimenting with various free headtracking software and hardware trying to find something that works. I've found that a great alternative to things like TrackIR is to use your android or windows cell phone's gyroscope data to track head movement.
You can find the full details of how to do it here:
https://github.com/opentrack/opentrack/wiki/Smartphone-HeadtrackingI should point out that you look pretty silly with a cell phone strapped to the top of your head, but if you live alone or are comfortable with being made fun of by significant others, children or roommates you can enjoy the benefits of head tracking while saving up for TrackIR or similar product.
