Ultrasonic aromatherapy diffusers atomize cool water and essential oil into a fine mist.
Think of a cool mist humidifier only shrink it down much smaller. These ultrasonic diffusers won't burn your essential oils. They usually have timers incorporated into them to let you adjust the saturation of aroma you desire. They work well with both thick and thin oils. You can place them on a side table as they don't take up much space.
Ultrasonic diffusers are also silent and safe because there are no glass tubes to be cleaned as with nebulizers. They also add much needed humidity to the air and are easy to clean.
I personally own the Aroma-Sonic model offered by Diffuser World shown below. It is quiet and does a great job in my living room which is large and has a cathedral ceiling! It has different timer settings built in, plus adjustable mist output, and auto shut-off when the water level drops below a certain level.