XLAuditor

The CubicleSoftTM XLAuditorTM add-in for Microsoft Excel 2000/XP/2003 enables professionals to audit, understand, analyze, create, modify, compare, and debug any formula in any Excel spreadsheet.
If you use Excel frequently, you know about the F9 key and the built-in Excel evaluator, but this tool is neither of those.

Take a good look at the following screenshots to see how you can safely look at Excel formula segments, perform deep analysis, and then edit just the problem area of a formula all from one convienent location.