Millions of people using Excel don't get why they see the "circular reference" error message right after they've entered a formula. The message means that your formula is trying to calculate its own cell--kind of like when a dog chases its own tail. Because so many of you (millions) searched on "circular reference" on Office.com, we thought we should very clearly explain how to remove or fix your formula.

We show you how we used an ActiveX list box control and some VBA code to allow our users to enter multiple choices in cells.

For those of us who crunch words instead of numbers, Excel can be intimidating. We don't know the difference between a workbook and a worksheet, and for sure don't know to ask about conditional formatting (a cool way to visually display data). Then one day your boss asks you to create a report--with numbers. Gulp.

In this video, the Office 911 emergency responders show a beginning Excel user how to add a table to a worksheet so she can better organize and view her data.

It's still January. There's still time to share the most popular posts on the Excel blog in 2011. Thanks for reading them!

The first one is--you guessed it--about keyword shortcuts.  From the post, you can download Quick Reference Cards to pin to your corkboard.

15 Spreadsheet Formatting Tips

For the last couple years I’ve been meaning to pull together some of the tips that I’ve learned working on the Excel team about how to make nice looking spreadsheets. Well, last week, Rob Collie (a previous Excel Program Manager, and now CTO at Pivotstream and author of PowerPivotPro.com) beat me to it with his post “In the Browser, Aesthetics Yield a Greater Return.”

You've finished your business plan and polished the pitch for investors. As an entrepreneur you know what comes next: number-crunching. To clinch funding, you need a financial projection that shows investors the path to profitability. We worked with @Guy Kawasaki to guide you through creating a financial forecast that will wow funders.

If you need a way to create invoices in Excel, you can use one of the many invoice templates on Office.com. While these templates are a great starting point, they don't automatically generate unique invoice numbers—you need a macro for that.

In this video, Excel MVP Bill Jelen shows you how to create a couple of macros to generate invoice numbers. The first macro automatically increments the number and then clears cells on the worksheet so you can start fresh on the next invoice. The second macro lets you save a copy of the invoice with a unique file name.

Need a plain-vanilla calendar for your business which shows the year in a glance? Or the kind that your local hardware passes out for free (marketing) at the end of the year?

Free 2012 Excel calendars are now available.

Suppose you want to send someone a demo workbook, but you don’t want that workbook to be used more than a certain number of times, perhaps because you want to be paid for your work. In today's VBA tip, Excel MVP Bob Umlas shows you how to use SaveSetting and GetSetting statements to set a limit on the number of times a file can be opened.

One of our Excel MVPs—Jan Karel Pieterse—emailed us this week suggesting we take a look at the online tutorial he created about Excel Mashups. We did, and now we want to share it with you.