Redscape Audio USB Head Tracker - 3D Headphone (includes software)
This thing is great, I just never get around to actually using it. Includes all originals and box as well as the software download. This runs $200 + shipping new from Redscape. More info here.
Basically, it uses a sensor group to determine relative head position while wearing headphones, and it basically makes it sound like you’re listening to speakers. With the right set of headphones and amp (I’ve used it extensively with Sennheiser 58X and 6XX) it truly sounds out of this world. It’s really great with movies, first person shooters, and music too.
Asking $145 shipped