Textmaker Viewer is a lightweight, fast, document viewer. It contains no editing functionality, but does allow you to open, search, and copy text from a variety of common document formats, including .doc, .docx (Office 2007), .rtf, and .odt (Open Document format, used by OpenOffice.org.)
"It does what it says on the box," as the saying goes. I was able to easily open a variety of documents. Textmaker Viewer has a few useful tools to aid in this, such as auto-stacking windows, view scaling (zoom in on a document, show it in 2-page mode, and so on), and a reasonable search feature (no regular expressions, but few business users know about them or are likely to miss them).
In addition, there's a Properties window that gives you word counts and other data for the whole document or a selected section. You'll also find a print feature and an e-mailer.
Why use a program like this? Sometimes, you are sent documents in formats you can't read--Office 2007 when you have 2003 installed, for instance. Some new computers might not have Office installed. Or you might not like Office. (Imagine that!) Or you might not want to bloat memory firing up Office just to read a memo or manual. Basically, if you've ever received a document file in a format you can't read, Textmaker Viewer might be for you--just be sure to check the supported formats (most common modern ones, but not a lot of truly ancient formats like Wordstar or Wordperfect) to be sure it will work. It's free, it doesn't take up a lot of disk space, and if it's useful even once, it's worth the download.