Like standalone proprietary mail software, Google Labs has experimenting many features to publish on internet for free. When time is of short, it is very helpful to get a quick preview of an email to check if the mail should be read or left unread.
Google Labs has recently provided the preview pane option to preview your mails. Here i have listed how to enable this feature in your gmail account.
How to Enable Preview Pane in Gmail
Click on the setting button at the top right to go to Google Labs Screen. Search for Preview Pane in Google Lab and enable for your google account
It will enable Preview pane in your Gmail. Now check your inbox at the top right hand corner for toggle split-screen button, Clicking on it once it allows you to preview your emails without opening
Explore more on Google Labs to see what Google experimenting on GMail.