Google's document viewer has a new interface that's more consistent with Google Docs. The viewer added a menu bar, a button that lets you save the file to Google Docs and a button for sharing the document with your contacts. The View menu includes a new feature called "compact controls" that hides the navigation bar and the header.
Google Docs Viewer is only useful for PDF files, Microsoft Word documents, PowerPoint presentations and TIFF files. It's integrated with Google search, so you can quickly preview PDF files without installing Adobe Reader or another PDF reader. There's also a Chrome extension that previews documents in Google Docs Viewer.