It is an angle of view calculator. It allows you to enter your format (a wide range of them) and it gives you the 35mm/FX equivalent as well as the horizontal angle of view. It will accept a number of lenses and compares the angles of each. Also of interest is a photo it uses to depict the angle of view. Very interesting when I compared the angles of view of my 20mm vs. 600mm Nikkors!
Lens Angle Calculator
Lens Angle Calculator