I installed a humidifier on my forced air furnace a few weeks back and just want to pass on my praise for the product. 

In the past, I've used one of those humidifiers you can buy at Walmart which just heat and vaporize the water.  These products draw a lot of current and my hard water really scaled the heater pad and filters, required constant cleaning and filter replacement.  Also, it's inconvient to continually fill the tank with water, and it doesn't do very well to humidify the whole house.

The furnace humidifier I bought is a Desert Springs model, and I'm really impressed with it.  It is installed in a bypass around the furnace blower.  Inside the humdiifer is a water bath which is controlled by a float level valve and a series of plastic discs.  These discs rotate through the water bath and the furnace air blows across the top of the wetted discs.  It's very simple but very clever because it uses almost no electricity (just a little 24VAC motor to spin the discs), hard water is not a problem, and there are no filters to clean or replace.  You can get an optional flush kit that will automatically drain the bath periodically but I do it manually, this helps to remove the mineral deposits which form in the bath.  As well as a clever and simple design, it seems very well constructed.  It is durable and solid, not like typical cheap and delicate plastic crap that's become common in many products.

But best of all, with the higher humidity in the house, I'm more comfortable at a lower temperature and I no longer have lizard skin.

I installed one in my furnace as well, though it's not a Desert Spring". It is connected into the return side of the furnace and it's connected to special contact points inside the furnace that only turn it on when the system is calling for heat, not just when the fan is on. That way, only warm air passes through it.