There's an article in Business 2.0 called Beating Bill that attempts to show how Microsoft can be beaten, or at least fended off for a while. I pulled a couple of bits I found interesting.

Intuit, who've defeated Microsoft Money with Quicken. They're facing a challenge to their QuickBooks software from Microsoft's purchase of Great Plains accounting software. One of the things they've done is to add APIs to QuickBooks to let third party developers' applications exchange and build upon the mission-critical small business data stored in QuickBooks. Information about these APIs is here. They use COM to communicate with QuickBooks but the actual calls and data passed are encoded in XML. Intuit even supply a Java-to-COM bridge so that Java developers can interact with QuickBooks using this API

Quote from "Beating Bill": "I think opening up the product is the smartest thing Intuit has done in years," says David Farina, an Intuit stock analyst at William Blair. "If they pull that strategy off, I think it will be hard for Microsoft to unseat them."

Liberate, which uses Linux as the OS for its set-top boxes. Quote from "Beating Bill": If Cook's (Intuit) game is to outrun Redmond, Mitchell Kertzman's plan is to use Microsoft's strength -- its control of the Windows operating system -- against it.

I found this interesting in relation to Apple's recently released Xserve product. On the main page they talk about the "per user" or "Windows" tax. Quote: Xserve lets you eliminate the most galling expense in your department’s budget: the per-user "tax" you’ve been obliged to pay for using server software. Perhaps this will be a fruitfull place to attack in the future. Can Microsoft's great revenue engine be used against them?