With Sneek Peek you can preview any message in your inbox???read or unread???simply by right-clicking it. The preview pane will display part of the message, as well as buttons for scrolling through the thread???if there is one???and a couple for archiving and deleting:??
After activating Sneak Peak in Labs, either right??click on a message or use the handy keyboard shortcut ???h???, and the preview pane will pop up. You probably won???t be able to see the entire message you???re looking at, but it???s definitely enough to figure out if the message is important or if it should promptly be sent to your archive. You can use the ???j??? and ???k??? shortcuts to scroll through your list of messages, looking at a preview of each.
Via: Gmail blog