Gmail Labs has some of the coolest stuff. I try to keep it as clean as possible (Inbox Zero!) and I’ve shown you other small time-savers from Gmail Labs, like “Send & Archive“. This is along those lines.
Recently, Google made it where you could hide labels (if you’re not familiar with Gmail, labels are similar to folders) from the sidebar on an individual basis. I took that opportunity to hide most of mine and keep them out of the way. Now they’ve added a tool in Labs to let you hide any that don’t have unread items in them. This keeps them all out of the way for me, unless something drops in to one of them. Most of the time, those e-mails would go in my inbox too, but I have a few mailing lists and such that I have go straight to a label. This will alert me when those arrive.
This is certainly not as cool as “Send & Archive” (which I still love), but it’s a nice tweak nonetheless.