Call the GNU Scientific Library from Excel.
Download xllgsl.zip, extract all files, then open
xllgsl.xls. Start the Function Wizard and click on
Help on this function to see the documentation.
This is not does not cover all, or even most, of the GSL. The Complex Math module has full coverage. It has Polynomial evaluation and root finding. It is now very easy to add any Special Functions you might need to the add-in. Feel
free to check out the code using Subversion and roll up your sleeves. You will need to install the
xll library to do development.
The GSL developers and maintainers did all the Herculean work. The libraries were built from David Geldreich's fine VC++ port.