There's more than one way to skin the revenue cat
37signals.com
37signals.com
Which is true, but if you don't do it carefully it's a good way to make your company no longer so small or agile.
What would be really interesting is an article that discusses not just that it is possible add very different products to your business, but how to identify which of these types of new add-on products would work well with your core business, even if the business models behind each look very different.
E.g. the way buxfer guys created iPhone interface for YCNews and Reditt.. since they already knew how to develop iPhone interface it's just extending their skills to reach out to more users..