In addition to chsag's advice:
Are the iMac power cords a) permanently wired into the body of the iMac, or b) is the power cord removable?
If a), then the power fault should be traced from the wall socket to whatever other power supply unit is being used, eg. a power board; then on to the connections inside the iMac. However, I suspect a fault in one of the power cables. Test the wall socket with another device or appliance to see if you still get the 'tingle'.
If b), then try a different power cable, but, if that were the cause, then only the iMac would be affected - unless the rogue current is being transferred to the external devices via their connecting cables, eg. USB/firewire.
Whichever, get it seen to a.s.a.p, before the problem gets worse - or serious.