MarkT:
That switchmode power supply should be giving out 5.0V +/-0.1V if it was any good at all - looks like its cheap dangerous junk: bin it. You may have damaged the Mega (6V is the abs max for the ATmega microcontrollers).Give the seller negative feedback if that supply really is giving out 6.6V...
I am starting to think that maybe my multimeter may be wrong also, I have put together a 7805 and using 12V-5V from a computer PSU to give 7 Volts.
and the output of the 7805 is also 6,87 V without anything connected to it.
so maybe the switchmode power supply is working correctely?
but then, the MEGA micro should not heat up!
So im confused.