Is there a plugin for head tracking with a webcam, without using free-track or purchasing a $200 IR tracker?
Check out opentrack (free, https://github.com/opentrack/opentrack (https://github.com/opentrack/opentrack)): it works fine with PiBoSo's games (via freetrack protocol). I know many use it with various web-cam, but to be honest the results seem to be poor (from comments on opentrack github).
If you want good head tracking for a few bucks, go with EDTracker: I have one and it works like a charm, much much less hassle than a web-cam or IR tracking, better precision, essentially no drift.
The "open source" version (DIY) of EDTracker is here: https://edtracker.org.uk/ (https://edtracker.org.uk/)
The commercial version (pro) there: http://www.edtracker.co.uk/ (http://www.edtracker.co.uk/)
In 2016 they will have a wireless version: insta-buy for me.
EDIT: forgot to add, if you go for the DIY version, be sure to buy the one with the magnetometer.