If you're using an older (pre-Office 2007) version of Microsoft Office you may run into issues when people send you documents saved using the new Office 2007 format, .docx for example. While people have come up with some convoluted ways to open Office 2007/2008 documents even if you don't have Office 2007/2008, Microsoft is making this a lot easier.

They've released a compatibility pack for older versions of Office. Once the pack is installed, you can use older versions of Office to open up documents saved in the Office 2007/2008 format. If you're working with people that use Office 2007/2008 and you don't have it, this free upgrade is a must-have. You can get it at: http://www.microsoft.com/downloads/details.aspx?FamilyId=941B3470-3AE9-4AEE-8F43-C6BB74CD1466&displaylang=en