While Digitech Drop is excellent for tuning down, let me recommend buying Digitech's DT Whammy. Costs twice as much as The Drop, but it's basically 3 pedals in once.
The drop side of Whammy DT Allows you to tune down AND up all the way to an octave. When going up/down an octave, you have an option to include the dry signal (makes hell of an octaver pedal effect).
The left side of the pedal has harmonies. For example playing single E note and putting the harmony to 5th and you get E5 power chord, or you can make something bizarre, like adding 2nd from the below octave, so playing E note gives you a very dissonant sound with playing E and F# at the same time. On a cooler note, you can do epic unison bend-style sounds with the expression pedal while in harmony side.
The middle part of the pedal is the Whammy side, which allows you to pitch shift entire chords with the expression pedal, as you would with whammy bar, but without the worrysome effect of having to deal with Floyd Rose. :)
This is all of course if you play or intend to play your guitar outside of rocksmith. If you are going to stick to rocksmith, then the drop is the better option.
If you are interested in some sound samples of the pedal, let me know and I'll whip something up :)