Existing homebrew music engines

That got me curious as to how many there actually are. As I understand it, some homebrew NES music engines are intended for NSFs, music ROMs, and demos:

NerdTracker II
FamiTracker
Musetracker
PPMCK: Macro based

Some are specifically designed for games, with smaller ROM and RAM footprints:

FamiTone2
My own sound engine, which lacks a snappy name

Which am I forgetting?

Two examples I can think of left off your list:

1. Metalslime's sound tutorial from NintendoAge ("Nerdy Nights Sound")
2. The Japanese NSD.Lib (NES Sound Driver & Library)

I've written two of my own, and hacked a lot of the others at one point or another.

Did TheFox write a unique one for Streemerz?

I used Musetracker for STREEMERZ.

I've written two of my own, one "largely featured" and another "resource efficient" one.

Here's one by Gradual Games: http://www.gradualgames.com/p/sound-engine.html
There's DragNSF which isn't publicly released as far as I know.

I actually do think a lot of people end up writing their own, even if they don't release them.

Long ago, I made a song using the music engine from Mouser.

I got a copy of dragnsf, it has some good features I'd like to see in other engines, such as triggering sound effects from note events.

My engine does that for all notes in the drum channel.

I started to write a music engine, for a quick Halloween game. (A cute Bubble Bobble clone.)
Unfortunately, work happened, so it's unlikely to be finished any time soon.

The .asm file is simply labelled "psg"
It's nothing fancy, just bare-bones functions.

Attribute Zone has its own music engine, using a compiler written in Csound, using CsoundMML to input the score. It is very simple though and does not do much.