I bought a few of these as they were cheap for my home automation setup. You can find some on ebay at 50$ (+15$ of you want a case). The DS18S20 or DS18B20 temperature sensors (up to 8 can be connected) can be bought on many online stores or directly from maxim.
It has a basic web interface, email capability and can even do a little control via its 8 digital I/O (start a fan/buzzer if temperature is over xxDeg).
A flexible solution if you can do a little electronics.
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