Newegg offers the Rosewill USB Bottle Cap Air Mist Humidifier, model no. RU-101, for $10. This $10 mail-in rebate cuts it to $0. With free shipping, that's $20 off and the lowest total price we could find. It's compatible with 5-volt USB hubs and power banks, and purports to moisturize skin. Rebate expires May 4.