The Netwide Assembler (NASM) is an x86 and amd64 (x86-64) assembler designed
for portability and modularity.  It will output flat-form binary files, a.out
(Linux and *BSD), COFF, ELF32, ELF64, Mach-O, Microsoft OMF (OBJ), Win32,
Win64, as86 (Minix/Linux bin86 v0.3), LADsoft IEEE-695, Intel hex, Motorola
S-record, and a home-grown format called RDOFF.  NASM syntax is similar to
Intel's, but is less complex.  It supports Pentium, P6, MMX, 3DNow!, SSE,
SSE2, SSE3, SSE4.1, SSE4.2, XOP/FMA4/CVT16 (rev 3.03), and x64 opcodes, among
others.  It has strong support for macro conventions.

The port also includes NDISASM, a binary file disassembler which uses the
same instruction set as NASM.

WWW: http://www.nasm.us/