ffttools are intended to constitute a spectral sound-design toolbox. Most of them are inspired by image processing techniques as you can find in PhotoShop, for example. mdsp used the FFTReal class by Laurent De Soras that can be downloaded at Music-DSP.
WARP: This plug-in swaps a frequency band defined between the fmin and fmax parameters while leaving the side bands unchanged. Low frequencies are mapped to high ones and high to low. The resulting sounds are highly inharmonic. You can try to set the min frequency very low (i.e. 100Hz) and the max frequency to its maximum thus letting the very low frequencies pass through while swapping the remaining frequencies...
LEVELS: This plugin affects the spectral dynamic. It classifies the magnitudes in 3 regions: small, medium and big whose gains can be adjusted independently between [-20dB;+20dB]. Therefore by boosting or cutting the small magnitudes, you can either add liveliness or remove background noise. Just try it to hear how far you can change your sound's character.
KONTRAST: This plugin also affects the spectral dynamic. It scales the spectral magnitude histogram according to a transfer function defined by the min, max and center parameters with saturation of the levels below the min and above the max.
ROBOTIZER: This one just forces the phase to 0.0 thus removing life from your sound. No parameter except a gain.