User defined functions without using VBA.
 

SDF (Sheet defined functions) is a new add-in to MS Excel that
end-users can use to create spreadsheet functions without programming
in VBA (Visual Basic for Application). Until now the end-user must
program the user defined function (UDF) as macros in VBA.
The creation of a SDF-function is easy and straightforward. To create
an SDF-function the user needs only define function name, input, output
and is able to create an adding formula in Excel.
SDF-funtions are especially useful for people who use spreadsheet in
their work.

For more informations, see
http://www.itu.dk/stud/speciale/PQ2006/index.htm