IMHO, a much more useful tuning, with more potential than open D is open C: CGCGDE. Stick on a capo at the second fret and you are in D, anyway. The reason for it being a more useful tuning is that the major third is safely out of the way on the top string, unlike open D or open G, where the major third kind of gets in the way.

I use this tuning for a lot of Celtic stuff, particularly jigs. Brian Mcneill plays ragtime in this tuning, so it really is versatile.

Murray