It's popular to make fun of Excel in some circles, but it really is a nearly perfect tool for innumerable real-world business tasks. It's intuitive enough that newcomers can do some work with minimal instruction, and powerful enough that you can manage a half-billion dollar book of business with it.

More than any of the iterations of Windows, Excel is Microsoft's great contribution to software, and I say that as an old-school MS hater.

I think people too often forget that "useful" is the key ingredient in killer software.