Google and Zoho have gone on the offensive as Microsoft gets set to launch Office 2010, the next edition of the software firm's office applications suite that includes online versions of Word, Excel, PowerPoint and One Note.
The addition of Office Web Apps (OWA) strengthens Microsoft's position against the Google Apps and Zoho suites, which until now had something of a stranglehold on the online apps market.
A recent survey by Forrester of 115 US and European enterprises found that Google Premier Apps is supported by four per cent of organisations, while Zoho is supported by three per cent.
Microsoft Office 2007, meanwhile, is supported by 81 per cent of organisations, a figure that could grow with the addition of OWA.
Matthew Glotzbach, enterprise product management director at Google, encouraged Microsoft Office users to switch to Google Apps in a blog post today.
"If you're considering upgrading Office with Office, we'd encourage you to consider an alternative: upgrading Office with Google Docs," he said.
"If you choose this path, 'upgrade' means what it's supposed to mean: effortless, affordable and delivering a remarkable increase in employee productivity."
Glotzbach made the case for Google Apps by comparing the cost of the suite with Microsoft Office.
Google Apps will cost a business $50 (£33) per user per year, while OWA is available only to business customers that use SharePoint and are using volume licensed editions of the Office 2010 Office Standard and Office Professional.
Glotzbach also boasted of the document sharing capabilities in Google Apps, although Microsoft OWA now supports collaborative editing in Excel and OneNote, so his argument goes slightly awry.
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