The DHT11 and equivalent humidity sensors can be connected to an Arduino or ESP8266 easily to provide humidity data to a home automation system. The local pull-up resistor is critical: reduce it to ensure you get a reliable digital signal from the device. Connecting them over a long distance has proven tricky as there is a lot of noise on the data line. Even with the correct resistors and short runs I find they still provide bad data occasionally (drop or add a bit) so I also feed them into a simple filter: keep the last 5 values, sort them by value and then pick the middle one.
I use humidity sensors in the bathrooms to automatically turn the fan on and the heating element behind the mirrors when the bathroom becomes steamy.
An overview of the many sensors I've experimented with for home automation including my favorite under-floor strain gauge, through all the usual PIR, beam and contact sensors to some more esoteric devices like an 8x8 thermal camera.
Bluetooth sensing for home automation is a great proxy for people counting as it can detect and locate each cellphone in the house. iBeacons attached to tools, cars and pets can provide a 'find my anything' feature too.
Having at least one light sensor is critical for any home automation system that controls lightng. Lights need to be turned on when it's dark not at specific times of day, especially here in Seattle when it can be dark and cloudy at any time of day.
Microwave doppler sensors can be found in some alarm sensors but there are also available very cheaply as a separate component. They offer exceptional range but suffer from false triggers requiring a probailistic approach to people sensing.