One feature that is long been missing from Gmail is the ability to preview messages without opening them. It is already available in other email services like Hotmail and this feature makes mail reading faster. Though currently not available by default, you can now add this preview pane option in Gmail. Here is how you can do that.
Sign in your Gmail account, go to Settings – Labs, and enable the experimental Preview Pane tool. A new ‘Toggle split pane mode’ button will be seen in the top right of your inbox.
Just click the button and select Vertical Split or Horizontal Split; a third column will be enabled that allows you to read messages right next to the list of conversations.
If you don’t like it, you will easily be able to switch to the familiar list view by clicking the No split option. (via)