My aunt was a human assembler at NASA (2015)

6502 SpASM (6502 Spreadsheet Assembler and Linker)

Implementation of a Two Pass Assembler in C : The following implementation used Tries for fast in-memory opcode fetch, generation of intermediate files and symbol tables

OS Dev Tutorial 10 - Creating a new GDT and adding an Assembler to the toolchain, next up an IDT for fault handling, keyboard, mouse, timer and more

8086-assembler

Custom JVM Bytecode Assembler using ANTLR 4, OW ASM

Full-Featured HexEditor new release! Added assembler/disassembler functions using Capstone and Keystone engines

Writing an assembler

I've Been Building a 16-bit VM from Scratch over a Series of Videos, Most Recently Working on the Assembler and Assembly Language

Iced: a high performance and correct x86 (16/32/64-bit) instruction decoder, disassembler and assembler written in Rust