Here are the four most common guitar tunings, along with an example of a popular tune played in each tuning and the fingerings required to play G, C, and D chords in each tuning:

1. Standard Tuning (EADGBE)
    Example Song: "Wonderwall" by Oasis
    G Chord: 320003
    C Chord: X32010
    D Chord: XX0232

2. Drop D Tuning (DADGBE)
    Example Song: "Kashmir" by Led Zeppelin
    G Chord: 5X000X
    C Chord: X3X010
    D Chord: X5X070

3. Open G Tuning (DGDGBD)
    Example Song: "Jumpin' Jack Flash" by The Rolling Stones
    G Chord: 0X0000
    C Chord: X3X033
    D Chord: XX0230

4. Open D Tuning (DADF#AD)
    Example Song: "Death Letter" by Son House
    G Chord: 5X0000
    C Chord: 9X0090
    D Chord: 7X0070

Note: In TAB format, the numbers on the lines represent the frets to be pressed down, and the X's represent strings that are not played. For example, the G chord in standard tuning is played by placing the third finger on the third fret of the sixth string, the second finger on the second fret of the fifth string, and the fourth finger on the third fret of the first string. In TAB format, this would be written as:

```lua
G Chord: 
e|---3---|
B|---0---|
G|---0---|
D|---0---|
A|---2---|
E|---3---|
```
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