I slightly tweaked this recipe that I got from a friend. Here it is:
one box of powdered sugar
one package cream cheese, well softened at room temperature
one stick of butter, well softened at room temperature
one teaspoon vanilla extract
Combine all of the above ingredients until well mixed.
I like to use on carrot cake, red velvet cake, spice cake, chocolate cake... really just about any cake is made better with cream cheese frosting.