Open E tuning is used a lot in slide guitar. "Open D" is the same relative tuning, but every note would be down one tone from what is shown below! I much prefer using Open D and putting a Capo at the 2nd fret, but for some songs you wanna be in Open E anyways.

It's pretty easy to get going, just tune strings 5 and 4 up one tone and string 3 up a semitone.

Open E Tuning

String	6	5	4	3	2	1
Tuning	E	B	E	G#	B	E

Some songs using Open E Tuning:

Rolling Stones - Gimme Shelter

....???

Please suggest more on the forum... I'm having trouble thinking of them now, but there are many!