Not sure if this is in the right place, feel free to redirect me if it's not 
Can anyone tell me the purpose of the 27 Ohm resistor that seems to go between the XTAL1 pin and the crystal on the 16U2 chip ?
I'm happy to put it on my board, I just don't see what its for and I've never needed or used it on any other Atmel processor.
Thanks in advance,
Gareth
Who recommends this?
The processor data sheet is your best and most reliable source of information. Also, check for a specific Application Note from the manufacturer covering oscillator crystal selection and PCB design considerations. AVR example
yes, you're right, I shouldn't have used the word "recommended" (removed from the original post now).
I was going off of an old Arduino Uno circuit diagram however that was using an 8U2 so I assume a Rev 2 board. On the Rev 3 board with the 16U2 that resistor isn't there any more.
Still curious as to what it's purpose was though 
Someone thought it was a good idea, which may have had to do with the particular crystal or resonator selected for other reasons.
See the application note link I added to the previous post.
Please don't. That can make things confusing for other first time readers.
Awesome, thank you !
Fair point, noted 
It's there to reduce the drive power to the xtal. A low power xtal can be damaged by a high power driver circuit
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