Desktop browsers like Firefox and Google Chrome include a useful “Undo Close Tab” feature to help you re-open any tabs that you may have (accidentally) closed.
You can either use the keyboard shortcut Ctrl+Shift+T to reopen the most recently closed tab or right-click on an open tab and use the context menu as shown in the screenshot.
A similar feature also exists in the mobile Safari browser of your iPad.
To re-open a recently closed tab in mobile Safari, simple tap and hold the “plus” button – the one that you otherwise tap to open a new tab – and it should display a list of the most recently closed tabs.
Tap a page that you would like to restore and Safari would open it in a new tab without overwriting the current tab. Check this video screencast for a quick demo.
Tabs in the Safari Browser were introduced recently and the above would only work if you have upgraded your iPad to iOS 5. Apple has not implemented this in iPhone or iPod Touch probably because of the limited screen estate.