Outlook 2007 has a great feature that enables you to preview attachments within Outlook without having to start an instance of another application. Windows Explorer (in Windows Vista) has a similar feature. Both are very useful tools as part of an information management strategy — personally I use them a lot in conjunction with tagging and search folders to help organize documents more effectively than using normal folder hierarchies.
Both Outlook and Explorer hook into the shell preview handler  mechanism which means it's easy for developers to provide support for all manner of different file formats (and you can take advantage of the preview capabilities in your own apps too). In this way we provide support for XPS previewing in both Outlook and Windows Explorer by including an XPS preview handler as part of the XPS Essentials Pack.
You can grab a copy of the XPS Essentials Pack from the download site. For those of you running Vista, and so already have XPS viewing capability, the essentials pack will install the previewer but will not change your existing defaults for which application to use to open XPS files (for example when you double-click).
 You might also have come across this referred to iPreview which, AFAICT, was used to refer to an abbreviation of the interface name.
 I've been meaning to post on this for a while now, but never managed to make the time. So thanks to Bill for asking a question which reminded me...