I was adjusting my alarm clock and felt a faint vibration. I explored the space around it and found regions of varying intensity... as if the field was a flowing arc of vibrating air. While it is tough to describe exactly, I would liken the sensation to feeling a loudspeaker's vibration by holding your hand near it, only with no sound or visual feedback. It's kind of eerie feeling something tangible in the air where there was nothing before, but at the same time extremely exciting.
Here's what I think caused the field, and will be the source of many other "in the wild" detectable fields:
My alarm clock, which is a cheap Wal*Mart plug-in unit, has a remarkably noisy AC to DC power supply contained within it. One of the main components of this type of supply is a transformer, which changes 120 V (the stuff from my wall socket) to a more manageable voltage (probably 5 to 9 volts.)
They look something like this:Basically, the transformer converts electrical energy into a changing magnetic field then back to electrical energy. To keep things simple I won't go into further detail (wikipedia explains it a lot better anyway,) but the interesting bit is the flux leakage. See, ideally the magnetic field would be completely contained within the core of the transformer but in reality, transformers partially leak some of the changing magnetic field. This field loops outside through the air and in the case of very inefficient transformers, this flux leakage can be quite significant..
This kind of leakage is prevalent in our modern environment, and will likely be the source of further discoveries. I have begun exploring things around me in a new light, trying out my new sense.