Note: this post is from 2009, outbound links may be broken.
synthgeek releases sg-freakshift
synthgeek has released sg-freakshift, a free VST effect plug-in for Windows.
This is a not-quite-technically-correct stereo frequency shifter (with separate controls for the left & right channels) based on techniques seen on this web page: http://www.cim.mcgill.ca/~clark/nordmodularbook/nm_spectrum_shift.html
The reason for this not being technically correct is that doing this the right way would require a lot more CPU. This implementation only achieves true frequency shifting in a certain range of frequencies, while frequencies outside of this range get an effect closer to ring modulation, which is still interesting.
There is also a simple distortion effect and LFOs for modulation.
sg-freakshift is available to download as a freeware VST effect plug-in for Windows PC.