Introductory video for the TrackIR5 from NaturalPoint, shown in conjunction with ArmA2 by Bohemia Interactive.
http://trackir.dslyecxi.com
Definitely watch this in HD. It shows pretty much everything you could want to know about TIR5, with lots of in-game footage as well as a look at the TIR software and hardware.

Feel free to ask questions in the comments. I'll get them as I see them.

Note that there are two comments that I will outright delete as of June 29th, due to having addressed them about 50 times already and having grown tired of it.

The first is "this sucks you have to look away from the monitor". My response to this is that you have not watched the video if you think that's the case. Watch it again, or look through the comments and see where I have already addressed that question.

The second response is "you can use ALT it's just as good". My response to that is simply that no, it isn't "just as good", it's actually quite different. ALT forces you to use your mouse to control your view - TIR allows you to control your view with your head and use your mouse to aim/fly/drive/etc. It's pretty clearly different, and nobody who has used a TIR would say that ALT is the same.

Aside from those two types of comments, I'll approve pretty much anything.

My system specs are:
Quad core @ 2.66GHz
Geforce 285
4GB DDR3 RAM
WinXP (less info)