I've been on a DADGAD kick lately. For just sitting with an acoustic, it's my favorite "let's get weird for a half hour" tuning. In fact, I've got a guitar in my lap right now and I'm screwing around with the song Si Paloma by Sun Kil Moon.It works wickedly well for an acoustic guitar also. I had a 50(ish) page book on alternate tunings, and Drop D tuning always made it's way to my guitars. Fun tuning to play around with.