Depending on your needs, Math Minion can be either a powerful scientific calculator that understands units, or a sophisticated modelling tool that is better suited to science and engineering problems than traditional spreadsheet programs.
It combines a mind map style representation of the relationships between expressions, with a formula system that operates transparently on scalars, arrays and matrices. A hierarchical model system helps encapsulate calculations in logical and easily duplicated units and a flexible unit conversion system tracks the fundamental property types through calculations. This allows it to know, for instance, that some fathoms divided by a few days is in fact a velocity that should not be added to a kilogram.