Over at Mashable they've got a look at 6 games you can play in Excel. Working actual games. They've come via Cary Walken, a chartered accountant who made Arena.Xlsm, a Dwarf Fortress style RPG in a spreadsheet.
Check Mashable's list and Walken's blog for more (and download links if you actually want to play any of these) but here are the pretty incredible highlights:
A 3D Maze Game
3D Maze was created by Jordan Goldmeier, another Excel professional (as in it's his day job). He's even posted a tutorial on how to do this sort of thing if you really want to have go.
A hardcore(ish) RPG
According to creator Cary Walken, Arena.Xlsm is an RPG that features 2,000 enemies, 1,000 item combinations, 31 spells and four possible endings. In a spreadsheet.
This started when creator Andrew Werner was trying to come up with strategies for the board game. It ended as a fully playable version for up to four people. It uses the Visual Basic programming language which feels a little like cheating, but still: in a spreadsheet.
Another one from proper business types that should know better. Not only is this a playable version of the original but it comes with a stealth mode to look like a normal Excel sheet so you can play at work. I'd post a picture of that but it just looks like a spreadsheet.