Cinnamon helps prevent/treat diabetes

The inner bark of Cinnamomum
verum is sold as cinnamon. Not to
be confused with cassia bark.

"8 studies carried out on human volunteers clearly showed that blood sugar levels were effectively reduced after meals and also in between meals by including cinnamon in the diet" Healthy Pages

Recent studies are bringing to light properties for many spices which go a long way to explaining why they are so often used in remedies across the Asian world. Cinnamon, the inner bark of the tropical tree Cinnamomum verum, is no exception to this.

Used in the Western cooking mainly for sweet dishes like spiced buns and rice pudding (though I've also seen it as an ingredient in hash browns), cinnamon is more often used in savory cooking for dishes like Indian curries, where it is just one of a medley of different natural seasonings.

Make sure you buy good quality cinnamon, as some of the cheaper varieties may be adulterated or entirely substituted by cassia bark (from the related tree Cinnamomum cassia), which is both inferior so far as taste goes and does not have the same properties. On the other hand, cinnamon essential oil from the bark is not used for medicinal purposes. Cinnamon leaf oil is used in aromatherapy, but is really only suitable for professional use.

More about cinnamon health benefits on HMYG
More about cinnamon essential oil on Guide to Aromatherapy.