I'd also like to add that a pipe wrench or large adjustable wrench will work fine.
Not sure about your vehicle's history, but if it was in the salt belt at any point, that sucker can be on there TIGHT!
Usually I prefer a band wrench to hold the pulley steady. If it's too tight for that, you can get a tool that has these little dowel pins that fit in the water pump pulley. And if you still can't get it off, there's a tool that will let you use an air hammer as a sort of impact wrench to get the thing off.
I don't remember if its right hand thread or left hand thread, but you can tell which direction you need to loosen it by taking a loot at the angle of the fan blades. When running, the system is designed to torque itself. i.e. the force created by the resistance of the fan is opposite to that of the direction of rotation of the pulley, and will tighten it. So if you want to loosen it, apply force in the opposite direction.
Look at the fan. See it? Ok, now grab the blades and spin it so that it would be pulling air through the radiator into the engine bay. That's the way you want to turn it to get it off
