The new arduino ethernet makes everything sooooooo easy that it looks like magic. In the same board you get: an ATMega 328, ethernet conectivity, sd card
Only need a RTC (Real Time Clock) in order to have a full datalogger. I will plan to make another instructable about how to conect a RTC to this board.
You can connect to my own arduino server in order to see my server room temperature.
Step 1: The basic stuff
Lets me show how easy is to make a datalogger that saves sensor data in a SD card and allow remote access by web using any browser.
We will use three DHT11 sensor as source of data. Every DHT11 give us temperature and humidity data. They are really cheap (2€ iin ebay) and more or less acurrate ;-) : 2 Celsius degree and 5% of humidity. In the case you want more acurrate data, the DHT22 could be your election.
I apologize for using Celsius degrees. That is because I radicate in Spain. Changing the units to Fahrenheit degrees it is really easy.