AC Hacker has had luck with these units:
MultiLogger
He includes a short description of them in this post:
http://ecorenovator.org/forum/geothe...html#post32670
The tiny board can be rigged as a standalone, battery-powered temperature logger or as a slave to another master, transferring data and obtaining power from a USB (or FTDI) cable. It can log over 100 OneWire DS18B20 temperature sensors at once. For under $50, it's an awesome deal.
There are lots of other plug-and-play temperature loggers available in many different configurations. Whatever flavor of sensor you like can be used, waterproof or not, that can transmit data over wifi or bluetooth radio channels or store data on whatever media you like. You just have to figure out a few things and find a compatible board. Most other devices are more expensive than the card cited above, once the capability to log 3 or 4 sensors (or more) is reached.