To open a Word 2007 document without installing Office 2007, you either had to download the Office 12 Compatibility Pack or manually convert .docx to .doc format using any of the online conversion services.
The limitation with Office Compatibility Pack is that it still requires a previous version of Microsoft Office on your computer.
Not anymore. Microsoft recently introduced a free Word 2007 viewer that lets you view docx Word documents, even if you don't have any version of Word installed. You will still need to install the Office Compatibility Pack.
Other than Word 2007, Microsoft also provides a PowerPoint 2007 viewer for pptx presentations. There's no Excel 2007 viewer yet but you can convert xlsx to xls using Zamzar and then view the converted spreadsheet in Google Docs.
None of the Online Office suites (like Google Docs) currently support the new Office Open XML format of Office 2007.