You can monitor lots of different data points about a Linux system using SNMP, if you pick the correct MIB numbers (e.g. like .126.96.36.199.4.1.2021.9.1.2.1 on http://www.debianhelp.co.uk/linuxoids.htm).
Where is the code for each MIB test stored on a Linux machine?
Is it possible to add your own custom tests? (e.g. when a custom MIB is queried, run a python script and return the response)?