XDrill

From Sheetware:

XDrill provides powerful drill-down functionality for spreadsheets; it can tell you 'where that number (or graph) comes from'. Results can be shown in an easy to navigate results window, or in another spreadsheet, and because the results window is dynamically linked to Excel, you can do what-if analysis quickly with even complicated spreadsheets. XDrill moves seamlessly between worksheets, follows links automatically and can retrieve figures from your network or over the Internet. It can list all the inputs that affect a particular number and even pick up those easy to miss constants that people put in spreadsheet formulas.