Guitar String company DR have come out with a new line of guitar and bass strings which provide you with drop-down tuning

Medium DDT-10 (10, 13, 17, 26, 36, 46)

Big-Heavy DDT-10/60 (10, 13, 17, 36, 52, 60)

Extra Heavy DDT-11 (11, 15, 19, 32, 42, 54)
XX-HeavyDDT-12 (12,16, 20, 38, 52, 60)
Mega HeavyDDT-13 (13, 17, 22, 42, 56, 65)
DDT Bass guitar sets are available in the following:
Medium DDT-45 (45, 65, 85, 105)

Heavy DDT-55 (55, 75, 95, 115)
Extra Heavy DDT-65 (65, 85, 105, 125)
Medium 5DDT5-45 (45, 65, 85, 105, 125)
Heavy 5DDT5-55 (55, 75, 95, 115, 135)