fasm (fast assembler for the x86 and x86-64 architectures)

The flat assembler (abbreviated to fasm, intentionally stylized
with lowercase letters) is a fast assembler running in a variety
of operating systems, in continued development since 1999. It was
designed primarily for the assembly of x86 instructions and it
supports x86 and x86-64 instruction sets with extensions like MMX,
3DNow!, SSE up to SSE4, AVX, AVX2, XOP, and AVX-512. It can produce
output in plain binary, MZ, PE, COFF or ELF format.