Abbott Aerospace has a wide range of spreadsheets available for free download.
Many are intended for specific aerospace design applications, but some are for general purpose engineering applications, such as calculation of section properties.
A general purpose add-in of particular interest is AA-SM-99-001 Equations.xlam, a macro for display of equations for report writing in Excel 2007. This add-in provides two User Defined Functions (UDFs) which allow Excel equations to be displayed as text strings with cell references replaced by either numerical values or variable names. See below for simple examples: