Current passing through me from Macbook to iMac?
Usually, ground loop current is experienced on portables where often no ground wire is connected - just the neutral and live (or hot) wires. 



If you are using a grounded plug and feel ground loop current you should either have the outlet serviced or the computer serviced.

It's normal (and safe) for ground loop current to exist and feel lightly tingly on Macs with magsafe connectors and only DC power going in to them. It's harder to be sure an iMac or other device that has 110/220 AC power directly into the case, so I would rather be too cautious and have you get an electrician to look at the outlet if your Mac works properly on other outlets or a technician to look at the Mac if it leaks current no matter which receptacle you plug it in to.

You might be able to find a grounding issue and solve this yourself, but if you have any doubts as to safety, I'd rather you chose to get expert help rather than trust our guesses based on how things should work.

See:


Is grounding important?
Electricity coming through screws, USB, and headphone jack on 2009 MacBook
How can I avoid my MacBook Pro giving me minor shocks?
Is it bad that my MacBook Air is passing me electricity?


And note that all the MacBook questions are low risk situations where any iMac one would be a little more danger (potentially).