Many of the cheaper humidifiers come with a "filter" that also acts as a wick. It wicks up the moisture from the water in the bottom and the humidifier pulls air through it thus blowing out cool, moist air. Problems are numerous. Mold, finding the right size of filter, remembering to change the filter, cleaning lime scale off permanent filters, etc. Furthermore, many of these systems require the filter to already be moist when you start it in order to work properly. So every time you forget to refill it and the filter dried out, you would have to remove it, set it in water, and replace it wet. If you didn't do this the system will most likely just be an over priced fan for dry air, or, it will continue to partially wick moisture and dump it all over whatever surface you have it sitting on.
You see my complaints 
The ultrasonic humidifiers basically just have an element or pan inside the system that vibrates super fast and disperses the energized water into the air. No filter necessary. These systems run $10-45 more expensive than the filtered systems depending on brand, size of room it will work for, whether it heats, and other such options.
If you are in the market, I recommend going to a couple local stores and seeing what they have. Then go home with those brand names and models and look up reviews of the products. There are a surprising number of lemons when it comes to this appliance so it pays to do your homework.
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Lincoln, NE
0.1 Pastel, 1.0 Pastel het Pied, 0.1 Pied, 0.1 Cinn, 1.0 Black Pewter, 1.0 Woma (hidden gene?), 0.1 Yellowbelly
2.1 Normals, 1.0 Thayeri, 0.1 Thayeri X Alterna, 0.1 Whitesided P. catenifer sayi, 0.1 crazy cat, 1.0 husband