I've been using stevia (in liquid form, which is better than the powder) and honey. Some people have concerns about the processing or additives in some stevia products, but I find it works for me. Just keep in mind that honey does contain natural sugars, so it's important to monitor your blood glucose levels when using it.
I also add cinnamon to my coffee and tea because it tastes good and some people find it may help with blood sugar, though the evidence is limited!