If you're preparing a spreadsheet, you often want to watermark it with a 'draft' or 'confidential' stamp to show it's not the final version. Excel doesn't support this directly, but you can add one fairly easily with a little header manipulation.
As Excel Blog explains, inserting a PNG file into the header of your spreadsheet will achieve the desired result. (While you might not realise it if you don't regularly print spreadsheets, you can associate headers and footers with them, just as you can with word processor documents.)
Hit the post for more details and to download some pre-made graphics.
Tip: Add a watermark to an Excel worksheet [Excel Blog]