It’s definitely a work in progress, but reBoxed takes a novel approach to organising your Gmail—forcing you to pick who’s more important a few dozen times, then using your (and others’) answers to rank emails.
The webapp, coded over just three days by former Gmail engineer Gabor Cselle, pulls in your Gmail contacts and inbox feed (after your approval, of course). It then gives you an A-or-B choice of two contacts at a time, though you can also tell reBoxed that a contact is an automated sender or mostly unknown. After you’ve played this game for at least 20 contact sets, reBoxed searches to see if any other of its users have given the thumbs up or down to one of your contacts.
reBoxed is free to use, requires a sign-up and granting API access to your Gmail.