I thought someone else would chime in...
I put in an April Aire whole house humidifier. They make numerous versions based on the size of the house. Mine is fairly small.
One (sometimes optional) feature is the outdoor sensor that modulates the total house humidity based on the outside temperature. It also turns it off automatically if the temperature rises above 50-degrees. As the outside temperarture drops, you are more likely to get condensation on windows or walls where there is low circulation, and if you always maintain the same relative humidity, you could start to get mildew on the walls. Having the outside sensor means you rarely have to adjust the control once you've found your ideal setting.
The things require periodic maintenance - changing the evaporation panel, cleaning out scale, and dust/dirt that may accumulate and let things grow. Depending on the type, this may only be an annual thing, but may require more frequent attension.
Lots of companies make these things...April Aire has been around for awhile, and mine has been reliable. I have no experience with others...so can't steer you to alternatives.