Right humidity level in the room or other area is as important as temperature. Not enough humidity leads to dry climate and overall feeling. Indoor plants may suffer, you sleep quality may drop. In other hand to much humidity could damage materials, have effect on your health. Like everywhere – there should be some balance. In practice human feel better when environment is more humid than dry. Personally for me 50% to 60% relative humidity is best. But not all places naturally have good humidity. Especially when heating season starts, humidity usually drops and you need to thing of ways how to increase its level. Many people use wet towels or other passive ways. Other use humidifiers.
__loovee__ from instructables have built his own arduino based automatic humidifier which constantly monitor humidity level and adds some humidity when it drops. He used DHT11 temperature/humidity sensor to detect humidity level. In order to add humidity he used water atomizer which is basically piezo element that vaporizes water. Arduino/Seeduino stores the threshold level which can be changed. He made acrylic enclosure which allows to see through and actually looks great.