I just finished building an Arduino-based server room environment monitor using a kit from Sproutboard. It is a pretty nice kit for the cost. The kit plus the Arduino and ethernet shield from Sparkfun comes in at under $250.
For that $250 and a bit of manual labor, I am able to see ambient temperature, humididty, incoming A/C temperature, if the lights are on, if the door is open and whether there is water on the floor, all from a basic webpage. The system itself has a buzzer for an audible alarm if any conditions are outside your specified parameters (positive test for water on floor, ambient temp > 80, etc). I also built a quick script for my Nagios system that pulls data from the web page so I get email/SMS alerts as well.
The code is really easy to work with. If you have ever done any C/C++ work you should feel right at home. I spent some time modifying the code so the system works exactly as I want it to. Can’t do that with many off-the-shelf systems.
I liked the system so much that I bought several more to send out to our remote offices. Some of our remote sites have a “server room” that is little more than a storage closet. It will be nice to have hard numbers on what the temperature is in those rooms.
Looking downt the road a ways, I will probably be adding a single-chip smoke detector, and I might hook up an AC-powered relay plugged into an outlet that isn’t powered by our generator. That will allow me to get alerts when we lose utility power.
You can also adapt this same system to monitor your house, control a greenhouse, etc. The Sproutboard site has lots of ideas. Check it out.