Free FFT plugin package for spectral sound design toolbox.
One standout is WARP, which swaps a frequency band between the set fmin and fmax parameters while leaving side bands untouched. Low frequencies are mapped to high and vice versa, resulting in highly inharmonic sounds. Experimenting with the min and max frequencies can yield even more interesting results.
The LEVELS plug-in is another great option, allowing you to adjust spectral dynamic by classifying magnitudes into three regions - small, medium, and large - with gains adjustable between -20dB and +20dB. You can add liveliness or remove background noise simply by adjusting the small magnitudes, and the impact on your sound can be quite significant.
KONTRAST is yet another useful plug-in. By scaling the spectral magnitude histogram according to a transfer function set by the min, max, and center parameters with saturation of the levels below the min and above the max, KONTRAST can have a dramatic effect on your sound.
Finally, ROBOTIZER might be the most unique plug-in of the bunch. With no parameters except gain, it forces the phase to 0.0, effectively removing life from your sound. If you're looking for a robotic, mechanical sound, ROBOTIZER is the plug-in for you.