This simple app not only provides the formulas for the area of 22 - 2 dimensional shapes, but additionally provides the formulas for things like the perimeter AND calculates these from your input. Take a right triangle; if you input the lengths of two of the sides, Area Calculator will give you the length of the remaining side, the area, and the perimeter. It also does this with less common shapes, like the trapezium, cycloid, ellipse, and octagon. There are many more shapes that are included, and any regular polygon can be calculated using this app as well.
Compatible with iPhone, iPad, and iPod touch. In-app purchase for full use $4.99.