New in Labs: Hide read labels
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. To help out with this, we've made a Gmail Labs feature called "Hide read labels." Turn it on from the Labs tab under Settings and all your labels without unread messages will be hidden under the "More" menu. Labels with unread messages will automatically show up, unless you've explicitly chosen to keep them hidden.
This is particularly handy if you use your inbox as a to-do list where unread messages are the outstanding things you need to take care of. If you use that method along with labels like "Home" and "Project X," it's easy to see all your to-dos in context. With this Labs feature on, labels with outstanding to-dos will be bold and have a number next to them; everything else will be hidden in the "11 more" section: