Many guitar players use different tunings apart from the standard tuning. Use of nonstandard tunings is rare in classical guitar, and use of nonstandard tuning is much less common than standard tuning, yet nonstandard tunings cannot be said to be "rare", and casual and serious guitarists alike may want to try them out.
Most alternate tunings involve downtuning ("dropping") strings. Uptuning the strings is less common, partly because it increases tension on the neck. Strings can even snap if tuned up too high!
A few bands, especially Sonic Youth, are noted for rarely or never using standard tuning.
The most common alternate tuning is the dropped D (or "drop D") tuning. The lower E string is tuned down to a D. This tuning allows to play power chords on the fifth and sixth strings with only one finger, and of course allows for lower bass notes. This is most commonly used in heavier Guitar styles such as Metal and it's sub genres.
An extension of dropped D is to also drop the high E string to D. This allows for playing nice open chords. Another variation is to drop only the high string.
Another variation was done by Korn. As they use 7-string-guitars they tuned down the 7th string (B) to A. So it's actually rather a dropped A tuning, but I think anyone get's the idea.
Oddball Joni Mitchell tuning
[that Steven Stills used in Suite: Judy Blue Eyes]
E,E,E[octave higher],E [same E as fourth string]B, E. sounds crazy but it really works.
Open G is a relatively common alternate tuning. Like many "open" tunings, it is often but not always used with a capo. The first and sixth strings are tuned down to D, and the fifth is tuned down to G, resulting in a tuning of DGDGBD. A rare variant, used by Pearl Jam and Alan Horvath, is to tune the sixth string up to unison with the fifth, yielding GGDGBD. In either case, strumming all six strings open results in a G chord, hence its name. Similarly, any major chord can be produced by simply barring a fret. This can be used to allow casual players to play rhythm, or serious players to use interesting chord voicings that are impossible to obtain with standard tuning.
Open D, like all open tunings, produces a major chord (in this case, D major) when all strings are strummed. Its tuning is DADF#AD: drop the sixth, first, and second strings down two semitones, and the third string one semitone.
DADGAD (pronounced as a word: "DAD-gad"), one of the most versatile tunings, is named after the tuning of its strings. The sixth, second, and first strings are dropped two semitones to D, A, and D. Strumming all the strings open forms a Dsus4 chord; fretting the second fret of the third string (or muting the third string) produces a D5 chord, or D power chord. Most songs for DADGAD are in D major, or in G major with a capo at the fifth fret.
Open C Major
A tuning which is mainly used by Canadian guitarist Devin Townsend of Strapping Young Lad. Before he began to use a 7-string, he usually tuned his guitars to CGCGCE, by dropping the low E (6) to C and tuning in fifths from there. With a 7-string, the tuning is simply GCGCGCE.