Metasm – A cross-architecture assembler, disassembler, compiler, linker (2011)

[BuuQu9hu]

http://metasm.cr0.org/

[xvilka]

I recommend to take a look to radare2 [1] - reverse engineering framework, which also has an ability to perform assembling for some architectures (and supported architectures and file formats count [2] is way beyond mentioned on METASM's site).

[1] http://rada.re

[2] https://github.com/radare/radare2