There are those small humidifiers, but there are also these big beasts that many people use with large homes, IF they don't have a furnace mounted humidifier. This takes 5 gallons of water!
I've had a console type Emerson MoistAir similar to this one for the past five years or so. It has two large water bottles, and I fill one at least once a day. After winter I put it back in the box and store it on a shelf in my garage. It works really well, and it humidifies the entire (1,800 sf) house. I also have a small one that I keep in our bedroom year-round that is on only when we sleep.
It's kind of a pain in the ass to fill the bottle, but really it's not that much trouble compared to the sinus, skin, static electricity and furniture problems we had before we started using it. I buy new filters every year, and I've never had mold problems.