Gmail Preview Pane

Gmail recently released a new labs feature "Preview Pane". As the name suggests, it provides a preview pane to read mail right next to your list of conversations, making mail reading faster.

To enable "Preview Pane",

1. Go to Lab under Settings
2. Find and enable "Preview Pane"
3. Save changes

After you enable Preview Pane from the Labs tab in Gmail Settings you’ll see a toggle button in the top right corner of your message list, which lets you switch between preview and list views. After you enable Preview Pane from the Labs tab in Gmail Settings you'll see a toggle button in the top right corner of your message list, which lets you switch between preview and list views. Preview can either be a "Vertical Split" or a "Horizontal Split". By default there is a 3-second delay in marking a conversation as read after previewing it. If that doesn't feel natural to you, you can change the timing in the General tab of settings.

I feel this should be a permanent feature but of course with option to disable preview.

» New in Labs: Preview Pane | Official Gmail Blog