www.aspartame.org is for aspartame what www.watchtower.org is for the Washtowel Babble and Crap Slaveholdery. I would look just below it and find the first link that takes you to a different parent web site, and you are going to find that it has something like 93 proven harmful side effects.
To start, the sxxx breaks down at 30 o C (86 F). Your body is 37 C/98 F. The breakdown products are aspartic acid (which, without other amino acids, will ruin your brain), phenylalamine (safe unless you have PKU), and methanol. The amount of methanol is significant--enough to ruin your liver, kidneys, and risk blindness (especially if you are a heavy user). The methanol breaks down into formaldehyde (very poisonous), and then formic acid (resulting in acidosis, itself linked to many degenerative diseases and inflammation). The aspartic acid itself will excite the neurons in your brain to death, especially if you are even slightly deficient in magnesium (which most of us are). It also plays havoc in the endocrine system, which can cause other health issues. As for one of the 93 side effects, death itself is included. By all means look this up and see how many sites include the same information.
If you are looking for a sweetener, real sugar (preferably organic) is probably the most obvious first choice. Stevia is excellent for your health, but some varieties have a bitter taste. Saccharin is what I would recommend if you must use an artificial sweetener, though it could damage your liver. Splenda, or chlorinated sugar, will ruin your liver as well (more efficiently). Acesulfame K may cause cancer and damage the liver. Xylitol is recommended, but it can cause diarrhea in excess and still contains calories. But, I would most definitely avoid aspartame and its cousin monosodium glutamate.