Numbers is a really useful spreadsheet program that comes in the Apple iWork suite. I'm a big fan (I wrote the official book after all). I just stumbled across this great website called NumbersTemplates.com that offers a ton of free templates that makes Numbers even more useful.
Some of my favorites include.
If you are on the fence, don't be. The application suite costs $79, has a 16% market share, and reads and writes all of the Office Formats.