A complete rewrite of the NASM assembler under BSD License. is designed from the ground up to allow for multiple assembler syntaxes to be supported (eg, NASM, TASM, GAS, etc.) in addition to multiple output object formats and even multiple instruction set http://www.tortall.net/projects/yasm/
A86: finest assembler, at any cost under any terms, for Intel x86/compatibles. D86: screen-oriented program to troubleshoot buggy programs written for MS-DOS; freezes program states, shows values of registers, flags, memory; single steps forward or back. http://eji.com/a86/index.htm
A self-assembling open-source 80x86 assembler for DOS, Windows and Linux. It supports all 8086-80486/Pentium instructions with MMX, SSE, SSE2 and 3DNow! extensions, can produce output in binary, MZ, PE, COFF or ELF format. http://flatassembler.net/
The continuation of SpAsm project. It's a PE Specific Assembler for ReactOS, Win32. All in one Assembler, Linker, IDE, Debugger. http://betov.free.fr/RosAsm.html