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On Monday, Microsoft announced that its upcoming Office 365 offering is now available as a public beta, opening up the service to millions of users. Office 365 was announced in October 2010 and had been in a limited private beta since then.

Office 365 is the successor to Microsoft's Business Productivity Online Suite (BPOS), which provides cloud-hosted versions of Exchange Server, SharePoint, Office Communications, and Office Live Meeting to midsized businesses and enterprises. Office 365 offers the most recent versions of those products—Exchange 2010, SharePoint 2010, and Lync 2010—and expands the offering to include individuals and small businesses as well.

Indeed, small-business interest in Office 365 has surprised even Microsoft. "More than 70 percent of the people who signed up for the limited beta were small businesses, so it clearly strikes a chord," Microsoft Corporate VP Kirk Koenigsbauer said.

To further spur small-business excitement in Office 365, the software giant has also started a contest called Office 365: Ready for Work. Qualifying businesses can share their stories on Microsoft’s Office 365 Facebook page, and five winners will receive Office 365 for free for a year, $50,000 in advertising and business services, and a Microsoft executive for a day who will work at the winner's choice of charity.

Office 365 will be available in both Small Business and Enterprise variants, with the least expensive plan—web-based access to Exchange and read-only SharePoint—costing $2 per user per month. Unlike with BPOS, customers can sign up for individual accounts with Office 365 if desired; prices start at $6 per user per month.

As part of the public beta, Microsoft has also unveiled the Office 365 Marketplace, an online store for Office 365 add-on apps and services from third parties. There are more than 100 apps and 400 services available today, Microsoft says, and new add-ons will be added over time.

I've been using Office 365 since last October and will have a new write-up available Tuesday. In the meantime, check out my previous articles, With Office 365, Microsoft Gets It Right and My Hands-On Experience With Office 365, for more information. To sign up for the public beta, visit the Office 365 website.