Docx is the new file format of Word 2007 and this is the default format of Word 2007. This format cannot be opened in previous versions of Word unless you have the office compatibility pack installed. Incase you do not wish to install the compatibility pack, there are online converters to do the job of converting to .doc format. Here is the list of top 3 online converters.
1. Zamzar: Zamzar can convert variety of file formats and Docx is one of them. Select the file and format to be converted and enter your email id, the file will be converted and a link will be sent to your email ID for download.
2. Docx2Doc: As the name indicated, this service converts docx formats into doc files so that it can be viewed in Word 2003 or lower. The major disadvantage with this service its that there is a one hour delay in downloading the converted files to reduce website bandwidth.
3. Docx Converter: This is again a good service to convert Docx file and the output file is in html format so that it can be read in all systems. There is also a desktop widget for converting.
In case you just want to view a docx file, then there is a simpler way for this. Rename the extension of file from .docx to .zip and then extract the contents. Docx is basically a set of xml files. When you unzip the file, you get a folder named Word and inside that you can see a file named document.xml. To view the contents of the document, open document.xml.
If there are other services, please do mention it in comments.