Problem: Was having problems with our small server room overheating. We have a dedicated AC for the room but it was malfunctioning from time to time. In order to avoid server fail/over heating, we needed something that would notify us if the temperature in the server room went above a certain temp.
Solution: Found the TemPageR 3E from avtech.com and it does exactly what we needed. The device is about $200, has two temp sensors and a built in web server so you can literally plug it in and monitor almost instantly. The device acts as one sensor and comes with a external sensor to monitor a different part of the room(You can buy a 3rd external sensor for $25).  I have the box sitting by the servers to monitor that temp and the external sensor is set to monitor the output of the AC. Both have a threshold set and when that threshold gets met, I get an email notification warning me of a possible problem. For the price, this is a perfect unit to give you a little piece of mind during the weekends. It also comes in a rack mount version.
Links:
http://avtech.com/Products/ID/179.htm