Microsoft CEO Steve Ballmer launched the much-awaited Office 365 on Tuesday, after a beta program of about nine months, as the company responds — some critics say belatedly — to the rising popularity of cloud-based applications for collaboration and communication.

“Office 365 is where Office meets the cloud,” Ballmer said at an event in New York.

Office 365 is the next version of the BPOS (Business Productivity Online Suite) collaboration and communication suite. Among the main improvements in Office 365 is that its applications, including Exchange Online and SharePoint Online, are based on the 2010 version of their on-premise counterparts, while BPOS’ are based on the 2007 version. Office 365 also comes with Lync Online, which is an upgrade to Office Communications Online.

Also significant is that Office 365 offers customers the option to have Office productivity applications like Word and Excel either through Office Web Apps — the online version of Office — or through the full-fledged, on-premise Office Professional Plus 2010 delivered via a subscription model.

Ballmer stressed that Office 365 is designed for businesses of all sizes. More than 70% of the about 200,000 companies that beta-tested Office 365 were small and medium-size businesses, he said.

Officials demonstrated Office 365 capabilities, like the ability for multiple users to jointly collaborate and edit documents in real time, not only from PC browsers but also from mobile devices.

“Collaboration is critical for business growth,” Ballmer said.

Microsoft also announced that Bell Canada, Intuit, NTT Communications, Telefonica, Telstra and Vodafone are some of the partners that will resell Office 365 with their own services.

With Office 365, Microsoft is responding to an explosion in recent years of cloud-based enterprise collaboration and communication suites from a variety of large and niche vendors alike.