The source code for the sensor module used in iStat Server is available under GPL.
Download the Sensor Module source code
Getting started with iStat Servers list Device Remote monitoring iStat Server for OS X iStat Server for AppleTV iStat Server for Linux, FreeBSD & Solaris iStat Server for Windows Multiple servers and port forwarding Firewalls and iStat iStat terminology Ping Traceroute Macminicolo Sensors Module Settings