Google Docs now allows users to upload Microsoft Office 2007 formats .docx and .xlsx. This is useful not only for Office 2007 users but also for lower versions of office. If someone sends you a Office 2007 file, instead of using an online converter you can just upload it to Google Docs and download it as a .doc file.
Google Docs only supports .docx and .xlsx right now there is no word on .pptx format, i hope they will add this in near future. Zoho writer has this functionality from long.
You can use this for converting .docx to .doc format if you are stuck with older version of Office, alternatively Microsoft has a tool that will do the same (download link).