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.
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)
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).