A86: high quality assembler, for x86 processors. D86: screen-oriented program to troubleshoot buggy programs written for DOS; freezes program states, shows values of registers, flags, memory; single steps forward or back. Eric Isaacson Software.

16/32/64 bit assembler. It supports all 8086-80486/Pentium instructions with MMX, SSE, SSE2, SSE3, SSE4A, SSE4.1, SSE4.2 and 3DNow! extensions, can produce output in binary, COFF format.

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.

Lists assemblers, cross-assemblers, linkers, and librarians, where available, for a wide variety of operating systems and processors.

Information and tools for Windows assembly language programming.

An assembly language developed by Randall Hyde which can use high-level language constructs to aid x86 assembly programmer beginners and advanced assembly developers alike. (Wikipedia article)

Linker for DOS, Windows. Open source, Artistic License.

The continuation of SpAsm project. It's a PE Specific Assembler for ReactOS, Win32. All in one Assembler, Linker, IDE, Debugger.

An open source customizable c++ assembler project.

Visual assembly language RAD IDE with built-in FASM assembler. Designed for easy development of big assembly written applications. Self-contained, Fresh is written in Fresh and is fully compatible with FlatAssembler (FASM).