There are a handful of open source alternatives to Microsoft Office, but one named LibreOffice recently caught my attention because of new features that were added in a recent update.

LibreOffice 4.1 can now open even more obscure file formats, including: ClarisWorks, AppleWorks, Microsoft Words, and CorelDraw. This is in addition to being able to open all versions of Visio files, plus file formats you would expect from an open source Office Suite like Word, Excel, and PowerPoint.
http://www.libreoffice.org