Hi,
I use the PitchFactor personally to do this (among many other things!). PitchFactor will do drop tunings if you set the mix to 100%. It will drop down all strings in the same interval, so the ratios between strings will still be the same (so you can't, for example, go to open D without modifying the standard tuning of your guitar), so in other words all strings will shift with the same ratio depending mainly on the key you've selected for it to shift in. But otherwise you can drop it down for any interval from two octaves down to two octaves up.
