Today Google introduced a new Gmail Labs feature, which lets users hide labeled messages that have already been read, when browsing through labels. The ones that have been read can be found under the "more" menu.
"A lot of people want to see their labels in order to see which ones have unread messages, but they don’t want a long list of label names cluttering up the left hand side of their inboxes," says Gmail software engineer David de Kloet.