In the US the mains power runs at 115V, 60 Hz. With an oscilloscope, I can see this mains hum everywhere. It radiates from the AC power lines throughout the lab and the whole city. Appearently our bodies make great antaneas for 60 hz: touching the oscilloscope probe shows a 9V RMS @ 60 Hz. Like a nine volt battery! From my body!
I also noticed that the using the probe clip picks up more mains hum than just the probe tip.
I’m learning how to use the scope. I suspect it will become my main measurement tool.
Today I received a Tektronix p6109 general purpose probe compatible with my scope:
Prometheus Fusion Perfection 
Yup, this is one of the joys of analog electronics. I work with photosensors, and they have gains on the order of 2500 – Just imagine that 9V signal after running it through the amplifier! Completely swamps the actual signal if you don’t design carefully.
A oscilloscope probe is a specialized coax cable, meaning that there is shielding (the outer braid) right up to the tip. When you use a clip end, you increase the amount of exposed wire, which acts as an antenna.
Chances are if your lab has fluorescent lighting, it’s coming from there.
If you have any sensitive electronics, you may want to enclose them in a Faraday Cage. http://en.wikipedia.org/wiki/Faraday_cage