It calculates key properties of the Earth's atmosphere as a function of altitude.
Atmosculator is a freeware Mac OS X application, written using Cocoa & Objective-C, that calculates key properties of the Earth's atmosphere as a function of altitude using the 1976 Standard Atmosphere Model.
Atmosculator also provides the option of calculating properties under non-standard conditions - i.e. when the temperature differs from that of the standard model.
Although the values provided by Atmosculator have been carefully checked against the original U.S. Government source, their accuracy cannot be guaranteed, so please use this tool at your own risk.
(If the above link does not work, try: U.S. Standard Atmosphere 1976 which is linked from this page of the NASA Technical Reports Server. This link might also be somewhat useful: U.S. Standard Atmosphere 1976)
Atmosculator includes extensive Help on the basis of the model and a mathematical derivation showing how the properties are calculated.