When you need to compare data on two spreadsheets, here's a handy way to do it:

1. Open both spreadsheets
    
2. Make sure Excel's ribbon is maximized and visible; if it isn't, press the Ctrl and F1 keys to expand it
    
3. Click the View tab on the ribbon
   
   
4. In the Window section, click View Side by Side
   
5. If you have more than 2 spreadsheets open, a dialog window will open to let you choose which ones to compare; click the filename of the one you want, then click the OK button:
   
   
    
6. My favorite part: turn on Synchronous Scrolling to scroll the rows and columns on both windows using the scroll bar on just one of them:
   

Here's an example of the view you'll see:

The default Side By Side view is horizontal, but if that doesn't meet your needs...

1. Click Arrange All
   
2. Choose Vertical
   

Other Arrange options:

• Tiled: when you have more than two spreadsheets open, Excel will arrange them as tiles on the screen — some horizontally, some vertically — depending on how many spreadsheets you have open
• Cascade: Excel displays the spreadsheet windows with the title bars cascaded down the screen

Sources:
• Spreadsheets used as examples in screenshots: Wisconsin Public Library Service Data: 1996 - 2012
• Apples and oranges photo credit: Automania via photopin cc
• Hat tip to How-To Geek for the idea