SoundHack has a binaural filter, which processes a monaural sound file so that the resulting file puts the signal at a simulated position around the head, emulating stereo. The difference is easiest to perceive with stereo headphones.
Processing a file with the binaural filter
Choose Binaural Filter from the Hack menu.
In the dialog that appears, click the position you want the sound to originate from or enter an angle.
Figure 1: Binaural Position Filter dialog.
The head in the dialog is looking upward, so the currently selected position in the figure below would, for example, play the sound 60 degrees left of front.