From Knight Consulting:
Excel Overlay compares two Excel spreadsheets and displays the differences using a new and intuitive approach. It assumes both spreadsheets are similar to each other and once "overlaid" onto one another, the differences become plainly apparent and easily corrected. This tool was designed for speed and is the fastest Excel spreadsheet comparison tool on the market today. Most tools of this nature are both slow and non-intuitive. They spit out a vast amount of discrepancies in either two side by side grids or hard to reconcile summary spreadsheets. Excel overlay allows you to make corrections right within the tool itself after which it will build you a brand new, corrected spreadsheet. The new spreadsheet will be the combination of the two you were comparing and any corrections or changes you made within this tool. Version 9.0.1 added support for .xlsx extension.
The idea to simply overlay one upon the other is definately thinking outside the box, especially when compared to the cumbersome implementations of nearly all other tools that perform a similar function.
The only drawback is a lack of clearly identifying missing rows - however a missing row is implied if a large number of mismatches suddenly start appearing on a row then continue all the way down.
When comparing two similar Excel spreadsheets you can easily tell the difference between the two of them. The values that are different are highlighted in red and both of them are displayed in a cell, one above the other. What is neat, you select the differences you want to keep and save spreadsheet under a new name.
The drawback is that if the spread sheets are quite a bit different, having a number of different rows and columns between the two, Excel Overlay does not work well.