I'm wondering if it's possible to power a microcontroller (e.g. ATmega328) from a Ethernet RJ45 motherboard port. Unlike USB, there is no power line, all pins/lines seem to be data only.
I am guessing I don't have much hope?
Power over Ethernet (PoE) is a standard way of powering things up to a few watts or more from an ethernet cable. You need to inject the power if your devices don't already support it. It's commonly used on things like IP telephones.
It depends what you are trying to achieve. The standard for power devices through a ethernet cable is PoE (Power Over Ethernet) which allow you to drain up to 15.4W, but if what you are after is to power your microcontroller without having ethernet communications, and just harvesting the power from the connector, then it is a different thing.
The voltage levels on the twisted pairs are +/- 2.5V, and potentially you can use them to drain power, but don't expect to be able to drain much current (10-15mA).
The question has already been asked before, and I cannot phrase the answer better than this guy has done: How could I power an IC over the Ethernet?