I use Agave regularly in my coffee, and like it just fine. It’s sort of like honey, but doesn’t have quite as distinctive a taste, it’s just sweet. I’ve also used stevia, which I don’t really care for, if you use too much, it has a VERY strong background flavor, at least to me, that I find quite bitter. There is a new product on the shelves called Truvia, which is made from stevia, and it seems to be much better than the stevia I’ve used in the past. I’m not 100% sure if it’s pure stevia, but you might check that out, I had it on strawberries and it was pretty good.
Another alternative is Xylitol…that is a natural sugar alcohol made from bark and corn cobs. It’s a nice sweetener and is getting easier & easier to find. I don’t think it has any off flavors (like aftertaste), although I do find that it reminds me of sugar free gum!