Christian Budde has released Christortion VST, a freeware plug-in which excites the harmonics of the input signal.
It is also possible to invert the phase of each harmonic. This concept guarantees a very surgical control over the distortion produced.
Christortion is released as donationware.
As there is no information about Christortion available on Christian’s webpage yet, you can download the plug-in here: Christortion.zip (website no longer available).