Math-fi library written in C++ and boost library. This contains a C++ library with severals financial models (Monte carlo, Binary Tree, Black and Schools formulas) and a managed C++ wrapper that allows end user to use library within Excel.
Derivatives portfolio modeler XL is a powerful option strategy simulator using what-if scenarios. Requires Microsoft Excel 2003 or OpenOffice 2.0+. Employs Black-Scholes model, well documented code with scientific references.