Hajo's Headphone Enhancer is described as 'affordable Mac app for music enhancement and virtual 5.1 surround listening'. There are four alternatives to Hajo's Headphone Enhancer for Mac, Windows, iPhone and Steam. The best alternative is Boom 2. It's not free, so if you're looking for a free alternative, you could try Balance Lock or Razer Surround.
Spatial Sound Card replaces the USB box that comes with 5.1 surround headphones, so that you can have surround sound on regular headphones for free. Also includes a volume booster and psycho-acoustic bass boost.