An alternate guitar tuning in which all the open strings add up to a G major chord; sometimes called "slack open G" because all the strings which change, change downwards in pitch.
E -> D
A -> G
D -> D
G -> G
B -> B
E -> D
With light strings, the low E can end up flapping if you tune it down by a whole tone. Not everybody uses 11's, though I'm damned if I can imagine why not. Keith Richards is fond of this tuning, and he usually removes the low E entirely. Of course, Keith can afford to tour with thirty guitars. Those of us who lack roadies and great wealth need to keep that string there for the next song, when we tune the guitar normally.
Slack open G is among the more widely used alternate tunings; Keith used it on "Start Me Up", "Brown Sugar", "Honkey Tonk Women", "Street Fighting Man" (or no, wasn't that was some kind of an open D thing?), and a number of others; whoever's playing the steel-string on "Moonlight Mile" is using it too. Jimmy Page used it on "That's the Way" and probably others. A better Led Zeppelin scholar than I could probably fill out the list a bit.
I can't vouch for this, but I've read that slack open G has been called "Sears-Roebuck tuning". The top five strings of it are a standard banjo tuning (see "How to tune a banjo"), which was transferred to the guitar in the 1920s or thereabouts when rural banjo players in the American South started mail-ordering guitars from the Sears-Roebuck catalog.
Like all open tunings, it's really fun to play with: You can fret only one or two strings, and let the open strings ring along. This makes for a big sound, and you can play melodies on some strings while letting others ring along as a drone. In concert tuning (EADGBE), there's usually something weird getting in your way when you try to do that. Of course you need a capo to change key, and that's a drawback, but guitars are arbitrary anyway.
Open tunings are frequently used for bottleneck playing, for reasons which should be obvious to a guitarist and stultifyingly arcane to the layman.