Closed down a tab you needed in Safari? In the latest version, you use the same keyboard shortcut you'd use to undo an oopsie in any other program to bring it back. That's right — Command+Z (Control+Z on Windows) to the rescue.
Firefox/Chrome: Small netbook screens make you really appreciate every last pixel on your device. The Mini Wide userscript for Gmail adds a "netbook mode" shortcut to Gmail that removes navigation and ads for a streamlined, netbook-friendly, message-focused view of your inbox.