I am looking for a small, cheap, simply device, that can measure temp and
be polled via SNMP. POE would be a plus.
Hmm... small, cheap, simple, pick any two?
If it's small and cheap that are most important, you could try building
your own like this:
Of course, that's not so simple unless you have some experience with
getting custom PCBs made and you have some sort of UNIX server at each
site to act as the SNMP agent. But it is about the smallest and cheapest
that you will find.