The Honeywell HIH6130 RHT sensor is an awesome little I2C sensor that operates on 2.3-5.5 VDC. Sparkfun makes a breakout board for around $30, but you can also make your own, as I did! Using Upverter, I threw together a quick PCB without alarms; you can easily fork the project to your own account and add those in if desired!

Here is the design:

HIH6130 by hagandh a3b0c6fa730e94e7 - Upverter