GHS Heavyweight set, originally developed for Zakk Wylde.
 
GHS says:
 
Since 1975, GHS Guitar Boomers™ have been THE POWER STRING, and continue to set the standard for musicians in nearly every genre of music. Nickel-plated steel wound over a round core brings a bright tone with a powerful attack, that lasts for an extended period of time. Each string is individually sealed in the GHS Nitro-Pack envelope (with our Anti-Corrosion Guarantee), ensuring they're as fresh as the day we wound them at our factory in Battle Creek, MI.
 
These sets were developed specifically for lower tunings. They feature a much heavier gauge/core wire combination that will allows your guitar to go lower without losing tone.