Mario Guardigli shows how easy it is to monitor environments using Arduino boards with Ethenet shields and various sensors. Apart from temperature and humidity, his system can also monitor door opening/closing, and leave open the options for other types of sensors such as smoke, vibration, light and so on. The Arduino operates as a web server and the data is monitored by Zabbix (an open source distributed monitoring solution). And to save money the housings for the Arduino have been made out of PVC foam sheeting that is somewhat fire resistant.
Building this type of monitor is much cheaper than commercial solutions, and allows for expansion and experimenting with ease. To get started with your own data centre monitor, start with our Freetronics EtherTen - the Arduino Uno compatible with onboard Ethernet, microSD socket and optional power-over-Ethernet support: