I once made an automatic humidifier controller (aka a humidistat). It consisted a remote-controlled outlet, a RF circuit, a temperature/humidity sensor, a humidifier, a screen, and a AVR microcontroller. The AVR microcontroller is programmed to loop, sleep, and read the measurements from the sensor periodically. If the humidity is below my threshold, it fires up the RF transmitter and switch on the outlet hence the humidifier connected, vice verse.
The remote-controlled outlet is a commercial off-the-shelf product for ordinary consumers. I reverse-engineered the 433 MHz signal using a RTL-SDR receiver and some widely-available programs. Using this outlet instead of making a circuit by myself allowed me to bypass the regulatory issues, as the outlet itself is already UL-certificated - no physical modification at all, if the outlet blew up anyway, I'm not liable for the damages at school.
A simple little thing, but extremely useful. It did a good job for maintaining a prefect humidity in my dorm room and made us survived the dry winter (RH 10 days of battery life, and fit on a single breadboard. All I need to do is refill the humidifier with water every morning.
The only trap is the unreliability of the humidity sensor, because humidity measurement is inherently hard. >15 USD sensors are reliable, don't attempt the project with cheaper ones, as the only humidity-sensing element they use is a resistor and it was completely unreliable. It was pretty frustrating and lots of fruitless debugging involved because of the crap sensor initially.