The Google Docs Viewer used in conjunction with Gmail is a great way to view attachments without having to save them to your computer's hard drive. Now, it supports more file formats. Additions include common file types like Microsoft Excel, PowerPoint, and more technical types like .EPS and .PSD files.
The newest additions round out support for all major Microsoft files and also what Google says are the most requested image files. Look for the "view" link at the bottom of a Gmail message with a supported type of attachment. You can also upload these types of files and share them via Google Docs.