Sound localisation lecture note from Washington U.
More details in part 7 of Introduction to Psychoacoustics and Sound Perception
2010 CCRMA workshop.
Introduction to HRTF.
About ambisonics.
Vector Based Amplitude Panning (VBAP) is an extension of stereo panning to an arbitrary array of speakers.
Spatial audio processing application.
Sound Spatialization in Pd from the 2007 Linux Audio Conference.
The SoundScape Renderer (The website also includes many useful links to current research on this topic)