Yasm is described as 'complete rewrite of the NASM assembler. It supports the x86 and AMD64 instruction sets, accepts NASM and GAS assembler syntaxes and outputs binary, ELF32 and ELF64 object formats'. There are three alternatives to Yasm for Linux, Windows and Mac. The best alternative is NASM, which is both free and Open Source. Other great apps like Yasm are flat assembler (Free, Open Source) and JWasm (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.
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.