nasm - The Netwide Assembler.

The Netwide Assembler (NASM) is an assembler and disassembler for the
Intel x86 architecture.  It can be used to write 16-bit,
32-bit (IA-32) and 64-bit (x86-64) programs.

NASM is considered to be one of the most popular assemblers for
GNU/Linux.

NASM was originally written by Simon Tatham with assistance from
Julian Hall, and is currently maintained by a small team led by
H. Peter Anvin.