Open tunings are cool, but I never really mess with them. They are very useful if you are using a capo or slide because you can just move that open chord up the neck. Open E and open G are pretty common. You basically just tune every string so you are forming a major chord triad. You can also do cross-note tunings which basically are just open minor tunings, but called cross-note for no reason. Then you can tune with modal tunings, or to any chord you want really. I've seen people tune all but one string to an E, and all kinds of weird shit.