GMail Priority Inbox
Gmail puts yellow arrows next to messages it believes, based on Google algorithms, you'll find important, and you can modify its choices manually (by clicking on the arrows) to help teach it what really matters to you. The visual cues save you time in sifting through your mailbox, but you can speed things up even further with Gmail's Priority Inbox feature. Just click on Inbox in the left nav bar (to bring up the inbox-style menu) and choose Priority Inbox, which will group at the top of your inbox all unread messages with yellow arrows. If you want to go back to sorting mail according to when it arrived, just pick Classic from the Inbox style menu.