yogurt is a pretty good source of calcium, but if you're concerned about sugar (and aspartame) you should check the labels, some of them are pretty high! especially the flavored kind.
also, if you are interested in increasing your calcium intake make sure you are not taking too much at once and that you're getting enough vitamin D so that your body can absorb it! i've heard that vitamin C can help you absorb calcium. also, don't take calcium supplements at the same time you take iron or high doses of zinc, they can interfere with each other.
but as someone else pointed out, it is always better to try and get your nutrients the natural way!
and make sure you're not overdoing it on the calcuim (usually want about 1,000 mg/day, not more than 2,000mg/day) it can have adverse effects such as kidney stones, etc.