I found this on allrecipes.com: http://allrecipes.com/Recipe/Easy-Chocolate-Mousse/Detail.aspx
Change it up by trying other pudding flavors.
1 (3.9 ounce) package instant chocolate pudding mix
1 1/2 cups milk
1 (16 ounce) container frozen whipped topping, thawed
Prepare the pudding in a large bowl using 1 1/2 cups milk instead of the suggested 2 cups.
Fold in the whipped topping until as well blended as you desire. A marbled look is very pretty to serve. Refrigerate until chilled and serve.