Windows/Linux (Firefox): Microsoft, through its open-source hosting site (yes, that really exists), has released a plug-in and extension set for Windows and Linux versions of Firefox that can open Office 2007 files natively. While there are certainly many work-arounds for those encountering the newer, won't-open-in-older-Office-suite .DOCX files, including using Google tools, converting by email, and others, Linux users in particular may find using Firefox a lot more handy. Both flavors require installing a plug-in and then adding the Firefox extension, and may also require tweaking Firefox's application settings. The OpenXML Document Viewer is a free download, works for Firefox on Windows and Linux. OpenXML Document Viewer[Codeplex via Digital Inspiration]
