A buddy of mine wants to monitor the temperature and humidity levels (among other things) in his dog kennels. I connected a Ethernet Shield to an Arduino, added a DHT-11 Temperature / Humidity Sensor, and whipped up a sketch that runs a web server on the Arduino, serving up the temperature and humidity to a users web browser. You could web enable any device in your home in this fashion! For the outside world to see the device, you would have to enable Network Address Translation (NAT) on your home router, and subscribe to a Dynamic DNS (DDNS) Service, but this is not that complicated.
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Connect the DHT-11 as shown above, using a 10k resistor as a pull-up between data and +5v.