Another new Gmail Lab feature is out today. Now you can preview Google Docs from inside Gmail. To enable Google Docs previews, go to the Labs tab under Settings and select “Google Voice player in mail”.

As per Gmail Official Blog:

Starting today, you can preview the contents of a Google document, spreadsheet, or presentation right in your Gmail inbox — just like you’ve already been able to do with YouTube videos, Yelp reviews, and Picasa and Flickr albums. Gmail will automatically detect when you receive a document link and display the name and type of doc below the email. Just click "Show preview" and the contents of the document will display right there — no need to switch back and forth between email response and document.

Previously, Google has enabled preview for Picasa, YouTube, Flickr and Yelp.