A partial capo, Shubb and Kaiser both make one, at the second fret on the 3rd, 4th and 5th strings puts a standard tuned guitar into EBEABE (i.e. one step up on DADGAD). This can give you the same 'Is it Major? Is it minor?' ambiguity when you require it, while at the same time maintaining standard chord shapes for any barre chords above the capo.

Additionally by varying how much of a first position D chord shape you hold down (i.e. by either fretting the 1st string or leaving it open)you also have access to tunings which approximte both dropped D and double dropped D - although they are actually in E. Effectively access to 4 tunings without actually retuning a string.

You can even move it into other keys by using a second 'full' capo 2 frets below the partial one, I've seen Steve Earle do this.

But, as several previous contributors have noted, it's what you do with any tuning rather than the tuning itself which is where the fun is. 'Celtic' melodies fall easily under the fingers in DADGAD and are fun to play but it's a tuning that lends itself to other styles of music too - try blues.

It would be a shame if people were discouraged from experimenting because a particular tuning was thought to have become hackneyed from over association with one particular style of music.