Windows: With Office 2010 already available to corporations, documents you can't quite open in Office 2007 are soon to appear in your inbox. If you're not upgrading, or never use Office anyways, TextMaker Viewer is a light, snappy document viewing option.
TextMaker Viewer actually opens a huge range of word processing files, including Office, OpenOffice.org, the TextMaker app this viewer derives from, and all the standard low-tech formats like HTML, TXT and RTF. Microsoft offers its own viewers, but only the PowerPoint Viewer has made it up to 2010 compatibility so far. Beyond that, TextMaker is a very lightweight application, so opening huge files likely won't choke up your system as thoroughly.
TextMaker Viewer 2010 is a free download for Windows systems only. There does seem to be an issue with registering the free version to turn off the "nag" screen on launching, but after a few starts, a checkbox to hide that screen does appear.