Aspartame is 200 X sweeter than sugar and the sweetness lasts longer. These properties and the innaccurate claims of it being a weight loss enhancing chemical accounts for its popularity. It is present in a huge variety of foods, or can be added from packets of nutrasweet or equal. Some of the many food it is present in include: 

Soft drinks, instant breakfasts, breath mints, cereals, sugar-free chewing gum, cocoa mixes, frozen desserts, gelatin desserts, juices, laxatives, chewable vitamin supplements, milk drinks, pharmaceutical drugs and supplements, shake mixes, tabletop sweeteners, teas, instant coffees, topping mixes, wine coolers and yogurt.  

So avoid these sources completely if possbile