John Miller, one of my favorite musicians, supposedly called Drop-D "God's own tuning" and I agree. It's extremely versatile, you can use it to make effective arrangements of fiddle tunes, Irish and Scottish dance tunes and airs, blues, jazz standards (Miller's done a lot of them),gospel,blues, Calyso and probably a lot of other stuff. Joseph Spence, the great Bahamian guitarist, used it exclusively.
I'm not sure what kind of music you're into but some good jumping off points for you might be Willie McTell's "Statesboro Blues" and Pete Seeger's "Living in the Country" which a lot of budding fingerpickers cut their teeth on. Good luck, frankie