I have plugged one end of an LED into the ground port of an Arduino.
By touching the other end of the LED with my finger, the LED lights up dimly. 
I can keep the LED lit as long as I am touching the LED, but it goes off if I also touch the USB port housing (ground) on the Arduino with my other hand. It comes back on when I let go of the USB port housing.
This implies that my body is somehow being kept at a positive voltage with respect to the Arduino ground and that the positive voltage cannot be discharged by touching Arduino's ground. But I don't know what is supplying this energy. I'm just sitting on my couch with my feet on the floor. The floor is made of wood and lifting my feet of the ground doesn't change the effect.
Any clues what is happening here?
Best Answer
This is probably AC leakage to ground trough your body.
You can measure this with an AC voltmeter, it often reads half the mains voltage.
This is not dangerous.
It is caused by the Y-capacitor in switch-mode power supplies used to prevent emitting lots high frequency of noise.
Example, CY1 2.2nF:
The solution to this is a grounded AC outlet. Secondary DC negative is then connected to ground, removing the weak AC component on the secondary.