flat assembler is described as '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!' and is an app in the Development category. There are four alternatives to flat assembler for Windows, Linux, Mac and Haiku. The best alternative is NASM, which is both free and Open Source. Other great apps like flat assembler are Easy code (Free, Open Source), MASM Builder (Free) and Yasm (Free, Open Source).
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.
MASM Builder is a free Integrated Development Environment IDE for developing 32-bit Windows programs using the Assembler. MASM Builder contains many useful features which will assist you in creating Windows API based programs.