The Microsoft XML Paper Specification Essentials Pack, Version 1.0 (XPS Essentials Pack) contains components that enable you to view, generate and index XPS Documents.
With the XPS Essentials Pack you can quickly begin reading and generating XPS Documents. It contains XPS Viewer EP to read XPS Documents and the Microsoft XPS Document Writer to output files to the XPS Document format.
Additionally, the XPS Essentials Pack download includes providers to enable the iPreview and iFilter capabilities found in many Windows applications. Windows shell handlers are also included to enable thumbnail views and file properties for XPS Documents.
Read more about it and download it
here.
Noteworthy for Windows Desktop Search and Sharepoint users:
Because it comes with iPreview and iFilters for XPS, you can install these on your SPS 200x indexing server and the contents of the XPS documents will be searchable. Ditto for users using Windows Desktop Search or Vista's built-in search.