Track IR
I just perchased TrackIR Pro 4, about a month ago, and I'll never go without again, it improves your situational awareness, and gives you a true feeling of flying.
My only issue with it is the high price I paid, which was well worth it, but my TrackIR suddenly stopped working couple of days ago, so I've sent it back and will get a replacement shortly.
I cant play without it anymore, and anxiously await its return.