The Netwide Assembler, NASM, is an x86 and 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 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.

Also included is NDISASM, a binary-file disassembler
which uses the same instruction table as NASM.    

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