To most people, Excel is the work tool that it is supposed to be. But not to everyone. Cary Walkin, a Chartered Accountant and Excel power-user, has built an Excel-based role-playing game: Arena.Xlsm. It is an impressive demonstration that entertainment is indeed possible with spreadsheets. This Youtube video gives a quick overview of the gameplay: http://www.youtube.com/watch?v=Egcp01OGPLE. You can download the game on Cary Walkin's blog.
But it does not stop there - countless other games have been implemented with the help of Excel. A quick Youtube search reveals many.