This is very interesting.
I have been using bitter gourd (Momordica charantia) that was recommended to me by an Ayurvedic Physician practicing in a remote part of India.
I have used it on my patients and on myself to control type II diabetes.
The blood sugar levels always show a remarkable decrease post administration.
The best method of administering it is as a juice from the fresh fruit but tablet concentrates do also show results.
Off the top of my head, I think Gymnemna sylvestris (sp.?) is useful to control blood sugar levels. And the general adoptogenic quality of eleutherococcus senticosus (siberian ginseng) is useful. In TCM try looking up the herbs used for "Wasting and thirsting" disorders. Theres been some studies done, but I cant think of any that specifically lower blood glucose off the top of my head (although I know that they are there.)
Oh, I forgot to add, somebody just posted a note about bilberry and anthocyanins, which has also been very useful in the treatment of diabetic retinopathy in particular, as well as peripheral neuropathy.