Well, yesterday I was starting to work with x86-64 assembly with NASMX, and it turned out that the "Assembler" language file that Notepad++ uses is out of date and only supports highlighting for old non-64bit x86 architectures. So looking online, I also found nothing, sadly; not even one language file. So I took a few hours and whipped up my own rudimentary one 
It supports every single current x86-64 instruction, with intel syntax. It also has some slight extra coloring specifically aimed for NASMX's directives. This first version can be found here: http://www.mediafire.com/?0rexibohf342lyh
If you see that I missed anything, do tell
And, lots of thanks to Kerm, Jonimus, Benryves, and Tari for helping me with learning x86-64 assembly, and for pointing out all the things I was missing in this file
It supports every single current x86-64 instruction, with intel syntax. It also has some slight extra coloring specifically aimed for NASMX's directives. This first version can be found here: http://www.mediafire.com/?0rexibohf342lyh
If you see that I missed anything, do tell
And, lots of thanks to Kerm, Jonimus, Benryves, and Tari for helping me with learning x86-64 assembly, and for pointing out all the things I was missing in this file