The flat assembler is a fast and efficient self-assembling 80x86
assembler for DOS, Windows and Linux operating systems.  Currently it
supports all 8086-80486/Pentium instructions with MMX, SSE, SSE2, SSE3
and 3DNow!  extensions and x86-64 (both AMD64 and EM64T) instructions,
can produce output in binary, MZ, PE, COFF or ELF format.  It includes
the powerful but easy to use macroinstruction support and does multiple
passes to optimize the instruction codes for size.  The flat assembler
is self-compilable and the full source code is included.

WWW: http://flatassembler.net/