VMware aspires to be for data centers what Microsoft is for PCs, both providing a software platform to run web applications (like, for example, Google Docs) and also managing the hardware that powers them.

FORBES: VMWare Sees Big Business In Becoming The Internet Operating System

"We are betting big on the cloud with our most successful product, and we are investing heavily both in the product engineering side and the physical infrastructure side, " Tim O'Brien, Microsoft's senior director of platform strategy, said last year when the company first unveiled a Web-based version of its Office software.

CNN: Microsoft's Office 365 available for public testing