Low-cost Arduino Ethernet webservers are reliable for different sensor monitoring: temperature, voltage or switches. In a pdf file is a description for Arduino webserver powered via the 5V USB connector or using Power Over Ethernet (POE).
Data can be fetched from Arduino Webserver webpage for example with ZABBIX that is a platform for IOT signal monitoring developed for server room monitoring and for sending out alarms. It is similar to Xively, but ZABBIX provides installation download free of charge http://www.zabbix.com
Power over Ethernet was a tricky thing to achieve. Ethernet connector of ENC28J60 shield has a transformer inside that galvanically separates inputs from outputs. To get POE the metal shield of the connector was cut off with a diamond disk allowing to solder wires to the connections before the transformer.
Attached pdf file contains description how to build versions without or with LCD screen.