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.
The site then rolls all that ranking together to provide a prioritised view of your inbox right from the reBoxed site, with the more important contacts stacked on top (though, in my screen pictured at right, I've only got one unread email this morning, and it's an automated bit of bacn). Some of the choices, obviously, are Hobson's-level hard to make and make you worry about the effect on your mail sorting. But since none of this effects your actual Gmail inbox, you can try it out and see how a little prioritising would re-rank your email.
reBoxed is free to use, requires a sign-up and granting API access to your Gmail.