Google's new Priority Inbox feature not only sorts your inbox, but it also figures out what's important based on what messages you reply to and read.
Gmail has one of the best spam filters around, but your email isn't made up of only good and bad email. After the spam is filtered out, you end up with mail you want or need to respond to, in varying degrees, as well as bacn—the email in between spam and a real message, such as advertisements and newsletters you've signed up for. Priority Inbox not only prioritises your bacn, but just about everything in your inbox. It watches how you interact with certain types of messages and senders, then shows you what's most important based on that information. If something doesn't look right or Gmail makes a blatant mistake, you can adjust message priority to make it work better for you. Also, if you don't want to deal with an important message right away, starring it will preserve it for later.
Priority Inbox is in beta and should be rolling out over the next week or so. When you see New! Priority Inbox in the top right corner of your inbox you'll be able to try it out.