- must have

Apps

Retroarch Aarch64 - your game launcher
RetroArch 32-bit - an older version of your game launcher
MIX - customizable file explorer, ebook reader, media player, text editor and more.
DIG - a game launcher with more looks and pizzazz when it comes to choosing systems and looking at your game titles

Single Console Emulators

DuckStation - Playstation1 emulator
AetherSX2 - Playstation 2 emulator
PPSSPP - PSP emulator
redream - HD Dreamcast emulator
Flycast - opensource Dreamcast and NAOMI emulator
M64Pluz FZ - Nintendo64 emulator
Citra Enhanced - Nintendo 3DS emulator (GitHub link)
Yaba Sanshiro 2 - Sega Saturn emulator

Streaming

Moonlight - stream games from PC to Retroid
SteamLink stream games from Steam website. not compatible with macOS 10.15 Catalina and above.

Multigame emulators

Openbor - free non profit games project. looks sick. definitely gonna explore this in the future.

ScummVm - Virtual machine for old RPGs and CD rom looking games, NGL this elvira game looks lit.

EasyRPG Player - a program that allows to play games created with RPG Maker 2000 and 2003

Single Game Engines

San Andreas Unity - an open source reimplementation of GTA San Andreas game engine, licensed by MIT.
Shattered Pixel Dungeon - Dungeon crawler RPG with randomized elements
Spelunky Classic HD - cave treasure hunting game with randomized play
OpenRCT2 - open source Roller Coaster Tycoon implementation? What? WHY?
OMW Nightly - an alternative opensource reimplementation of MorrowWind
PokeMMO - A free to play Pokemon MMORPG game
VCMI - open source engine for a game called Heroes of Might and Magic
Serious Sam Android (github site) - i think its the engine for a game called Serious Sam 2, part of a first person shooter that looks totally bonkers.
PojavLauncher - a Minecraft launcher for Android. I guess you can play older versions of Minecraft with it?
OpenLara - opensource engine for Tomb Raiders 1 - 4.
UNciv opensource reimplementation of Civilization.