Yes, many humidity sensors act as capacitors in a circuit. There are some more expensive "integrated" ones such as the sensiron parts. You can use the sensor in most any circuit that could use a capacitor. A simple example would be to build an oscillator with a 555 using the sensor to vary the frequency (and as a result the pulse width). Then trigger another 555 in monostable mode, you can just lowpass filter the output of the second 555 and there is your analog voltage.