Product review (4 stars): Low-tech Humidifier simple, safe, and never fails.
I bought this register-insert humidifier after three ionic, sonic, and otherwise high-tech humidifiers had failed in three years.  This one snaps into the register space in place of the normal grate.  A simple filter sits in a reservoir (nest to the air channel) and can be filled with water in winter, or left alone in summer.  The amount of humidification was not as high as a programmable, humidistatic unit, (the humidity rose perhaps 10%) but it was enough to take the edge off the dryness in my upstairs office. I would fill it once a day when it was semi-cold (32 F) outside, and 2 or 3 times when it was 10 F.  I think it holds about a pint at a time.  I would recommend something more heavy duty for a bedroom in a cold, dry climate as this is. On the other hand, this is one unit that Will Not Break Ever, unless you stand over the register and jump on the (plastic) great, or set heavy items on it.  Nor do you need to decalcify it, and you could buy just about any flat soft humidifier filter and but pieces from it. A kitchen sponge has worked too.  Highly recommended for home office.  Silent, too.