A86 is a great minimalist assembler for 16-bit code. In the humble words of its author:

"A86 (with its 32-bit version A386) is the finest assembler available, at any cost under any terms, for the Intel 86-family of microprocessors (IBM-PC, compatibles, and not-so-compatibles)."

Actually, come to think of it, I'm not sure if a86.exe is a 16-bit binary or 32, so it might not run on modern Windows.

A86 might as well be perfect, and I'd still not use it.

It is not Open Source.