Next to the Electro Harmonix NYC Big Muff, the Boss Hyper Fuzz FZ-2 is one of my all time favorite fuzz pedals, due to it's abrasiveness and all round versatility.
The Hyper Fuzz FZ-2 has three modes. Fuzz mode 1 is a dirty garage sounding, Rolling Stones 'Satisfaction' style fuzz. In terms of fuzz, this mode is tamer than the big muff, and would be suitable for garage rock.
Boss Hyper Fuzz FZ-2 Fuzz mode 2 is a very dry, metallic, industrial style tone. Think Downward Spiral era Nine Inch Nails, and that's probably getting close. There is a hint of ring modulation in mode 2, although not enough to distract you from the grind. This is were you would leave the pedal set if you are into industrial, heavy style music. For a good example of what mode 2 sounds like, have a listen to the Orgy cover of 'Blue Monday'.
The third and final mode is Gain Boost. This is probably the most surprising mode available on the pedal. When I've run the FZ-2 in Gain Boost mode in front of my JCM800, my amp screams. The level of boost the FZ-2 can provide is huge.
I can totally understand why some people have chosen to run two FZ-2's in their rig, one for fuzz, the other solely for a clean boost.
The Boss Hyper Fuzz FZ-2 has been long discontinued, but has built a cult like following. Fortunately, I picked mine up quite a few years ago off eBay, in near new condition for $35US, however I've seen them going for $80US plus recently. I'd recommend this pedal on the basis of the Gain Boost alone, but throw in the two modes of Fuzz, and this pedal becomes a "must buy". Get one before the prices go any higher.
Famous guitarists known to have used the Boss Hyper Fuzz FZ-2 include Stephen Carpenter (Deftones), Amir Derakh and Ryan Shuck (Orgy), Mike Sarkisyan (Spineshank), Robert Schneider (Apples In Stereo), and Robin File (Badly Drawn Boy).
