Ok, this is cool. I had read about this recently, but not actually looked at it myself.
If you are using Google+ and Gmail, you'll now see a new option on the left nav - "Circles" - that if you click it open you'll find all of your circles.
Click on a circle and it will filter your inbox just to show you emails from people in that circle.
This is slick - a whole new way to sift through your inbox.
This will be a big boon for companies you add to your circles in Gmail.