Humidifiers that are attached to your furnace offer a range of benefits over freestanding models, most notably that you only need one of them to humidify your entire house. Generally, these will be fitted when your heating system is installed as they are designed to be fully integrated. You could also have one attached at a later date if you can find a compatible model, though it should be noted that this is not possible for all furnaces.
As you can read elsewhere on this site, there are many different techniques used to increase humidity. Although you can easily find each method used in tabletop models, furnace humidifiers only tend to use two or three standard techniques. In fact, they are almost exclusively cool mist designs because the warm mist options are expensive to run and need more maintenance.
The few years ago, the most common option was the evaporative wick design. These used a sponge to soak up water, which is then passed into the air. Unfortunately, these sponges or filters need regular cleaning or replacement to prevent them becoming a breeding ground for mold and bacteria. These days, the impeller (rotating disc) is the most common in use. This design is effective, efficient and low maintenance. Combined with an automatic cleaning system that periodically flushes away any deposits that have built up, you're humidifier would only need attention a few times each year. Although they don't need cleaning often, it is important to stick to the maintenance schedule, so make sure you don't forget!
One thing that you should look out for is a built in humidistat. This allows you to set the required humidity level for your home and prevents over humidification. A system that is connected to your mains water supply is also vital otherwise you will have to spend a lot of time filling up the reservoir.
