The latest version of Opera (v30.x.x) introduces a new productivity feature to preview the content of other browser tabs without leaving the current one. You simply have to hover your mouse over the browser tab of your selection and a preview window will be displayed. This feature is disabled by default, so here's how to enable it:
Click on the Opera menu and go to Settings. Select Browser settings (in the sidebar) and scroll to the User interface section. Tick the checkbox next to Show tab previews and exit the settings window:
Tab preview is now enabled in Opera.
NB: If the Show tab previews option is missing from the browser settings, then make sure the latest version of Opera is installed. Click on the Opera menu > About Opera to check for available updates.