Open G tuning
The open G tuning is popular among guitar players who favor blues styles. It gives a slightly different sound than normal. The many straightforward movable shapes make some chords very easy to memorize and it also gives chromatic possibilities.
To get the open G tuning on your guitar you tune it like this: DGDGBD. The strings that differ from the standard should be turned so the string tension is decreased and not the opposite.
Including an octave perspective, the tuning is written D2-G2-D3-G3-B3-D4, meaning that the lowest string is a D note on the second octave, the second lowest string is a G note on the second octave and so on. Standard tuning reference: E2-A2-D3-G3-B3-E4.
Major chords (movable shapes)
All you have to do is to lay your fingers over the strings and avoid playing on the lowest string (this will not be incorrect, but the chord will be inverted in the way the bass note will be the 5th instead of the root). As the pictures illustrate, the G chord can be played with all open strings.
The shapes are movable, so if you move the finger two steps up the fretboard from D major you will get an E major.
All common major chords in open G:
F: X 10 10 10 10 10
Blues in G
One of the most common things to play with the Open G tuning is blues in the key of G. Put the slide on your ring or little finger and shift between G on the 12th fret and C and D.
7th chords (movable shapes)
The seventh chords are also movable and you just add another tone in front of the barre finger three frets up. To get a maj7 chord, just add the finger on the highest string one fret up.
Two ways to play a blues shuffle is using these building blocks (examples for C):
C: X5555X / C5: XX55XX
Minor chords (movable shapes)
While major chords can be extremely easy in this tuning when it comes to movable bar shapes, the minor chords are trickier. You probably recognize the Am-shape here, but besides the fingers in the Am-shape you add two notes on the bass strings by placing the thumb over both the strings. If you want, you can leave out the 6th string to avoid playing the chords inverted.
You could instead use open position chords, which in most cases are more natural for the fingers.
Sometimes the Open G tuning is modified to EGBDGB, which can be seen as an Em7 (or G/E) tuning and makes it easier to create minor chords whereas the 6th string isn't involve in several major chords.
Minor chords (open positions)
Em could also be played including all strings as 202002. Bm could also be played as an inverted two-finger chord as XX2200.
Sus and add chords (movable shapes)
The same method is used once again for sus and add chords. To get a Bsus4/Badd2 just move the whole shape one fret up the fretboard. The same thing with Badd2. Another movable chord related to these are sus4sus2. For example Asus4sus2 is played as X22432.
Other chords including open positions
Here are some additional open chords and voicings in open G tuning:
This was an introduction about chords in open G tuning. This video could be recommended if you want to learn more chords and ideas on how to apply them.
If you want to go more into depth of this particular guitar tuning, see Essential Chords in Open G Tuning with over 300 chord diagrams (ebook with pdf chord charts).
See also open D tuning and open A tuning