MicroTuner
MicroTuner adds microtuning capabilities to every VSTi and external synthesizer module, as long as the target synth supports receiving pitchbend on all MIDI channels.
I am not talking about changing a note some cents up or down in pitch, no, you can completely redefine the whole tuning scale, allowing you to set an arbitrary frequency separately for each MIDI note ! And it gets even better, it already has a loader built in (with drag and drop GUI) that can load the widely used Scala, VAZ and Anamark tuning files directly !