(Via Hack a Day.)
Like making ethernet-enabled Arduino sensors, but hate running wires to power them? Well, rather than building a complicated energy-harvesting system or relying on batteries, you might want to take a look at powering them using the same Ethernet cables that are used to transmit data to them. This can be a bit messy to implement, but luckily the folks at Freetronics have cooked up this Ethernet Shield With PoE to do the dirty work for you. Just plug it in, hook up a single power supply at your router, and you are good to go!