I am setting up a security camera mounted under the eve of my roof. Rather than running an ethernet cable and an extension cord up there, I figured I could use the four unused wires in an ethernet cable to provide the camera with power. The camera only uses 9v, so it should be pretty safe. I had a couple extra 1ft patch cables lying around, so I spliced the transformer into two patch cables. This will let me run a long CAT 5 cable from end-to-end, and attach it to the camera and ethernet/power.
Here's a photo of how I spliced the power into the ethernet patch cables.
And a close-up of the wires soldered together.
I'm pretty sure this is all safe, but if you try this, be sure to test all the connections thoroughly with a multimeter and CAT 5 cable tester before plugging it in to the wall or any computer or switch.