Aspartame, which is an artificial sweetener, has been linked to a variety of diseases, including headaches, depression,and cancer. Studies conducted by the industry always show this product's safety, whereas almost all independent studies have shown dangers. Even if this chemical has not been linked to diseases, it seems common sense to avoid putting chemicals in your body.
Although I try to avoid aspartame, sometimes I buy something that I think is healthy, only to discover that aspartame is one of the ingredients. Here's a chart for 10 sources of aspartame:
Yogurt and chewing gum are the two sources that I sometimes forget about. Cooking sauces and children's medicines are the two surprising ones for me.