Author: Wikipedia 

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Open Tuning"

Major open tunings (giving a major chord with the open strings) include 

Open A E A C# E A E
alternativly E A C# E A C#
Slide Open A E A E A C# E
Open C C G C G C E
Open D D A D F# A D
alternativly D A D' A' D D
Open E E B E G# B E (use light gauge strings because two strings must be raised)
Open F F A C F C F (rare)
Open G D G D G B D
Dobro Open G G B D G B D (occasionally adopted for ordinary guitar, but requires lighter fifth and sixth strings)

Crossnote tunings

The above open tunings all give a major chord with open strings. Since it is highly likely guitarists will need to play minor chords as well, open tunings must be adapted to allow this by lowering the pitch of one of the strings forming the open chord by half a step. To avoid the relatively cumbersone designation "open D minor", "open C minor", such tunings are sometimes called "crossnote tunings". The term also expresses the fact that, by fretting the lowered string at the first fret, it is possible to produce a major chord very easily.

Crossnote tunings include

Crossnote A E A E A C E
alternative E A C E A E (rare)
Crossnote C C G C G C Eb
Open D D A D F A D
Crossnote E E B E G B E
Crossnote F F Ab C F C F (extremely rare)
alternative F C F Ab C F (used by Albert Collins; requires extremely light gauges)
Crossnote G D G D G Bb E

Modal tunings

Sometimes a guitarist will want a tuning that will permit very easy chords but not be defintively minor or major. In this case, modal tunings can be used. They can be especially effective with droning open strings, and give "suspended" second or fourth chords:

Modal tunings include:

Csus2 C G C G C D
Csus4 C G C G C F
Dsus2 D A D E A D
Dsus4 D A D G A D (very popular in Celtic music)
Esus2 E A E F# B E
Esus4 E A E A B E
Gsus2 D G D G A D
Gsus4 D G D G C D

"Extended chord" tunings

These tunings allow a guitarist to play an open seventh, ninth, eleventh or thirteenth chord. One or more of the strings is retuned to the appropriate note of the required scale. Such tunings may be either minor or major.

Examples are:

Open Dmaj7 D A D F# A C#
Open Dmin7 D A D F A C
Open G6 D G D G B E
Dobro Open G6 G B D G B E (two lowest strings tuned up and require lighter gauges)
Open G7 D G D G B F (or F G D G B D)
Modal G7 F G D G C D
Open G6min7 F G D G B E
Open Cmin7 C G D G Bb Eb
Open C C G C G B E