At work we bought a couple of Ethernet Boxes from MessPC to keep an eye on our different server room and monitore temperature, humidity and the presence of smoke. These Ethernet boxes are quite neat: they have one network port (10 Mbps half/duplex only, beware) and four RJ-45 ports where you plug-in the sensors you need. We got sensors for temperature, humidity and a smoke alarm.
Once connected to the network, you configure everything (IP, sensors and snmp communities) through the web interface. Once everything is in place you can start monitoring your box via snmp. The temperature sensor is on physical slot 1 (port 1-4), humidity on slot 2 (port 5-8) and fire alarm on slot 3 (port 9-12). The OIDs are : .188.8.131.52.4.1.148184.108.40.206.1.5.x - where x is the port. The returned values is the respective measured value multiplied by 10
.220.127.116.11.4.1.14818.104.22.168.1.5.1 - Temperature
.22.214.171.124.4.1.148126.96.36.199.1.5.4 - Humidity
.188.8.131.52.4.1.148184.108.40.206.1.5.8 - Smoke
The temperature/humidity sensor:
Astrud Gilberto / Flying on the moon