Venn Audio says that Quick Haas is a free and simple psychoacoustic panning plugin, available as a Windows VST or mac Audio Unit. Here are the details in their own words...
The 'Haas Effect', also known as the precedence effect, is a psychoacoustic effect where listeners can perceive the special position of an audio signal depending on very small delays between the left and right channels of the audio signal. This allows for a much more realistic panning effect than traditional panning, provided the playback device is in stereo.
This plugin couldn't be simpler to use! If the input is stereo, just select how you want it to be routed into the pan (it is recommended only one channel, or mono, is used if the original source's left/right channels differ), and then adjust the central knob left right so as to make the sound appear to be coming from that direction. Hint: this plugin allows for very fine adjustments of up to a tenth of a millisecond, and often only small adjustments less than 2ms is all that's needed to create a powerful precedence effect!
Pricing and Availability:
Free download (donations welcomed)