For those of you that enjoy a large widescreen monitor and want to better utilize the space when you are reading emails, head over to Gmail Labs and enable the new Preview Pane feature — You’re going to love it!
Pretty much like in Outlook, Thunderbird, Windows Live Mail, or Gmail on tables, you can now split vertically or horizontally the view of your emails when using Gmail on a web browser. On the left side of the screen you’ll see a list with all your emails and a small hint of the initial text of the main message content, and in the right side you’ll see the content of a particular email.
Once the feature is enabled and you return to the inbox, you will notice a new toggle button in the top-right-corner of your message list that will let you switch between preview and list views.
Source The Official GmailBlog