The SHT21 is a low cost humidity and temperature sensor. Its an I2C device so again is very simple to connect to any arduino
The SHT2x sensors contain a capacitive type humidity sensor, a band gap temperature sensor and specialized analog and digital integrated circuit – all on a single CMOSens® chip. This yields in an unmatched sensor performance in terms of accuracy and stability as well as minimal power consumption.

UNO and SHT21
This is the breakout for the sensor that I bought.

HTU21
Here is a simple schematic, again be careful as Vcc is 3.3v with this device
Schematic
Lets look at the required libraries and a basic code example, there is not much to this to be honest most of the work is done in the library
Code
There are a couple of libraries to help you out
https://github.com/misenso/SHT2x-Arduino-Library
https://github.com/elechouse/SHT21_Arduino
I used the latter one
#include <Wire.h>
#include <SHT2x.h>
void setup()
{
Wire.begin();
Serial.begin(9600);
}
void loop()
{
Serial.print("Humidity: ");
Serial.println(SHT2x.GetHumidity());
Serial.print("Temperature(C): ");
Serial.println(SHT2x.GetTemperature());
Serial.println();
delay(1000);
}
Links
these come in at about $7 for a useful breakout board with this sensor
Humidity Sensor – SHT21 Breakout Board