It seems like the ThunderX chips from Cavium are the most prevalent 64-bit arm marketed as server platforms. Very high core count, high memory capacity.. I've been hoping that these things take off because I love the idea: http://www.cavium.com/ThunderX_ARM_Processors.html
System76 has a server using this chipset: https://system76.com/servers/starling
and Gigabyte has a whole line of them: http://b2b.gigabyte.com/ARM-Server/Cavium-ThunderX