Meshuggah.
...and I'm gonna mention Mnemic here. Granted, they tune down to F# on sevens, they're one of the few bands that use low tunings in anger, crafting whole songs around the low string... as opposed to a sprinkling here & there, or "It's time to drop to the low string for the breakdown".
I'm also going to mention a 7-string band that plays with a very low tuning...it basically sounds like they're rocking 8-stringers:
STRUCTURES (my avatar ) they rock hard, man.