Description:
Yasm is a complete rewrite of the NASM assembler under the "new" BSD License (some portions are under other licenses, see COPYING for details). Yasm currently supports the x86 and AMD64 instruction sets, accepts NASM and GAS assembler syntaxes, outputs binary, ELF32, ELF64, COFF, Win32, and Win64 object formats, and generates source debugging information in STABS, DWARF 2, and CodeView 8 formats.
http://www.tortall.net/projects/yasm/wiki/Download