MicrosoftFileFormats - Raymond Yee's Wiki

What type of open source libraries are there to handle Microsoft document files?

One is Jakarta POI - Java API To Access Microsoft Format Files:

The POI project consists of APIs for manipulating various file formats based upon Microsoft's OLE 2 Compound Document format using pure Java. In short, you can read and write MS Excel files using Java. Soon, you'll be able to read and write Word files using Java. POI is your Java Excel solution as well as your Java Word solution. However, we have a complete API for porting other OLE 2 Compound Document formats and welcome others to participate.

I first learned about POI from an article in onjava.com.

Microsoft's own documentation

Lots of buzz around the announcement that Microsoft is making publicly available (what exactly the terms mean is debated) the schema for MS Word 2003:

news.com article

From the slashdot thread, one gets interesting discussion of what the license restrictions really mean. One comment caught my eye in particular.