About latest XML document file formats
The latest document formats are based on the Open XML architecture, a zipped XML based file format for text documents, spreadsheets or presentations. Open Office XML is de facto standard for any modern documents used by many developer subjects and governments specified in ISO/IEC 29500:2008.
Both Microsoft Office documents formats, such as DOCX, XLSX or PPTX or OpenOffice.org documents formats, such as ODT, ODS or ODP are based on Open XML format, thus can be opened in any software that is capable to handle Open XML formats is able to work with them.
How to open latest document formats
To open and work with the documents you can try one of the following option depending on what you need to do with the file. If you need to work with documents on regular basis its probably better to get yourself whole office suite.
Use latest Microsoft Office or OpenOffice.org editions
The most obvious option is to open and work with the documents in latest Microsoft Office 2010, which supports all documents in Open XML format. Alternatively, if you already own some older edition of Microsoft Office and do not want purchase upgrade, you can download compatibility pack for latest Word, Excel and PowerPoint file formats from Microsoft Website.
If you do not want to play for rather expensive software like Microsoft Office just to be able to work with few files, you can get any of the free office suits such as OpenOffice.org or LibreOffice available for download on Internet.
While there will be probably no problem with opening OpenOffice.org formats in Microsoft Office, there may be some loss of special Microsoft Office features or formatting embedded or used in its documents, when opened in another office suite that does not support such features. However the actual text, spreadsheet or presentation data should not be affected.
Use one of the available viewers
If you do not want to work with the documents, just read or print them, you can download any of free document viewers available on internet. Microsoft offers free Word, Excel or PowerPoint viewers for download on their website. But, you can probably find many other viewers on sites with free/shareware software, or on website of office suite developers.
Use a document web service
You can also use one of the document services to open your documents, or even take advantage of cloud technology and web storage if you work with your documents daily. Popular document web services include Google Docs, Microsoft Office Web Apps or Zoho.