Truvia - you can buy it in most supermarkets. It's made from a natural leaf and, as they say, if it's from the ground - it's sound :-)
You can also buy pure Stevia, but you'd need to go online to get this. Truvia is just as good. If you really are giving up aspartame (excellent choice!) then be sure to look out for it in absolutely *everything* - especially where you'd least expect it. Yogurts, most diet/low fat products, calorie free drinks, table sauces (found it in a bottle of ketchup - switch for organic), chewing gum and sweets. I had to swap my favorite no added sugar Ribena for the 'full fat' stuff, as that also has it in, but I'd rather a few extra calories than something which is proven to be linked to cancer. Hope that helps :-)