I use this one. Actually, it's an older version of this one that was about $40 cheaper.
The Honeywell Top Fill Cool Moisture Tower Humidifier features a programmable LED digital humidistat that will provide you n accurate measure of the surrounding levels of humidity. This top fill humidifier features a 1.5 gallon capacity that's capable of running up to 24 hours.
www.honeywellstore.com
I use one of these in the basement, where my guitars are. It's about 350 sq ft. I'm in the Chicago area, and this keeps the basement between 42% and 50% through the winter months. Also use one in the living room, and one in the master bed room.