RTM is a big day for the Microsoft engineers who have been slogging away to get the software out the door. It's also a big day for Microsoft, which still gets a hefty chunk of its profits from Office, even as low-cost, Web-based alternatives from Google, Zoho and others gain in popularity.

In fact, some of the to Office 2010 are online. The software includes Office Web Apps, which are lightweight versions of Word, PowerPoint, Excel and OneNote that can be accessed through a browser from desktop PCs and smartphones.

Microsoft thanked the "more than 5,000 organizations and partners" who helped test the 2010 products. It said 7.5 million people have downloaded the beta of Office 2010 since its release in November, three times the number who downloaded the Office 2007 beta.