Yasm!

The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed for portability and modularity. It supports a range of object file formats, including Linux and *BSD a.out, ELF, COFF, Mach-O, Microsoft 16-bit OBJ, Win32 and Win64. It will also output plain binary files. Its syntax is designed to be simple and easy to understand, similar to Intel's but less complex. It supports all currently known x86 architectural extensions, and has strong support for macros.

The flat assembler is a fast and efficient self-assembling x86 assembler for DOS, Windows and Linux operating systems. Currently it supports x86 and x86-64 instructions sets with MMX, 3DNow!, SSE up to SSE4, AVX, AVX2 and XOP extensions, can produce output in plain 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 complete source code is included. »

JWasm is a Masm v6 compatible assembler. It's released under the Sybase Open Watcom Public License 1.0. JWasm can be used for virtually any purpose, unlike Microsoft's free download of current Masm versions, which are for non-commercial use only.