The DHT-11 and DHT-22 digital temperature and humidity sensors are great, low-cost sensors to monitor temperature and humidity. Both sensors have a 3-5V operating voltage. DHT-11 is an ultra low cost sensor, with less accuracy (humidity: 20-80% with ±5% accuracy, temperature: 0-50°C with ±2°C accuracy, 1Hz sampling frequency /read every second/), while DHT-22 is a low cost sensor with a better accuracy (humidity: 0-100% with 2-5% accuracy, temperature: -40 to 125°C with ±0.5°C accuracy, 0.5Hz sampling frequency /read every 2 seconds/).
For wiring up a DHT sensor, see this great tutorial.
We prefer DHT-22 for our setups for increased accuracy.