I'm putting a Mega2560 in a machine which has +5V power supply, the other power supply is +24V. Is there a way to feed that 5 volts to Mega. No HW modifications, just a pin or connector for +5V INPUT. +24V requires an extra chip, but +5V directly is better.
Hello
the best way to feed an Arduino with a voltage of 5 volt is the usage of a USB connector.
There is a pin labelled 5V, connect 5V to it.
Feed a well regulated 5V to the pin labeled 5V (next to the 2 grounds).
Don't power the Mega with USB and the 5V pin at the same time, though. If you need to be connected to USB, power the circuits external to the Mega with the external power supply. Connect the grounds.
if you feed modules through the Mega all power goes through those skinny little copper strips. you use your mega as a fuse. I use only Mean Well brand 5 volt switching supplies. unbranded power supplies from ebay tend to be counterfeit ripoffs.
I use dual row headers plugged into tiny business card sized breadboards as power and I2C proliferators. then they came out with this:
Very Good. Thank you.
groundFungus:
Don't power the Mega with USB and the 5V pin at the same time, though.
Let me clarify that! 
Do not power the Mega 2560 through the "5V" pin if you are connecting via USB to a PC. The reason is specifically that there is a danger that your external 5 V supply may be a significantly higher voltage than the PC's 5 V supply and feeding current back into the PC's USB interface may cause the USB interface to malfunction and has been reported to sometimes cause permanent damage, especially to laptops.
In all other regards, supplying 5 V to the "5V" pin is the only correct way to power it, and definitely not the relatively useless "Vin" pin or "barrel jack". Just arrange to disconnect the "5V" pin before you connect to USB to program it.
"Ground"s must always be connected. 
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