Technologist Ryan Lowe has created a very simple and useful remote thermometer system that allows monitoring temperature using the cosm (was "Pachube") system using an Ethernet-enabled Arduino. His reason for doing so was to allow a friend to check that their furnace was working, as it had failed in the past and allowed the cold weather to cause serious damage. This solution is perfect as they can now watch the temperature from almost any Internet-enabled device. The temperature is measured with the Analog Devices TMP36 temperature sensor, and read by the Arduino's analogue input pin. The data is then sent to cosm for recording.
Use the outstanding Freetronics EtherTen. Apart from being fully Arduino Uno-compatible, it has onboard Ethernet, microSD socket, full USB interface (so you don't need a costly FTDI cable just to upload a sketch!) and supports optional Power-over-Ethernet: