Luca Bolognese has released a .NET library that provides the full set of financial functions from Excel. The main goal for the library is compatibility with Excel, by providing the same functions, with the same behavior. Note though that this is not a wrapper over the Excel library; the functions have been re-implemented in managed code so that you do not need to have Excel installed to use this library.

More here including the download links: http://blogs.msdn.com/lucabol/archive/2008/12/04/financial-functions-for-net-released.aspx