Last Christmas I gave my father in law a weather station to monitor temperature inside their summer house to avoid the pipes to freeze during the winter. It has been working well but has only been measuring the temperature inside the house. During the holiday we added another sensor to be able to also read the temperature outside the house.
See this blog post for the original setup
Adding more thermometers is straight forward since the sensor used, a digital DS18B20, uses the OneWire protocol which the Raspberry Pi has drivers for. Adding more sensors doesn't require more ports on the Pi but instead reuses the same three ports for power, ground and data as the first sensor. Each thermometer has a unique hard wired address which is used to communicate over the data wire with the Raspberry Pi driver.
Programmatically, the new sensor is represented as a device folder in /sys/bus/w1/devices. You can simply cat this new file and find the latest temperature reading.
The Raspberry Pi posts the temperature readings to a CouchDB server and a small single page app presents the historical readings with graphs for the current day, week and month.
The original code is available on GitHub at https://github.com/johannoren/WeatherStation.
The adaptations for multiple sensors can be found on branch https://github.com/johannoren/WeatherStation/tree/multiplesensors