Yasm is a (mostly) BSD-licensed assembler that is designed from the ground up to allow for multiple assembler syntaxes to be supported (e.g. NASM, GNU AS, etc.) in addition to multiple output object formats and multiple instruction sets. Its modular architecture allows additional object formats, debug formats, and syntaxes to be added relatively easily.
Yasm started life in 2001 as a rewrite of the NASM (Netwide) x86 assembler under the BSD license. Since then, it has matched and exceeded NASM’s capabilities, incorporating features such as supporting the 64-bit AMD64 architecture, parsing GNU AS syntax, and generating STABS, DWARF2, and CodeView 8 debugging information.