An emulator system consists of two parts: the emulator cores, and the frontend. These can either be separate projects, or be part of the same project.

Combinations of cores and frontends can be distributed either as programs running on an OS, or as a dedicated OS of its own.

That said, here are the options available (mostly focused only on Linux):

Emulator Cores

DOSBox

Emulator Core Formats

libretro

Emulator Frontends

Launchbox [PROPRIETARY]
Playnite
Pegasus
EmulationStation
ES-DE, a maintained fork of RetroPie's fork of EmulationStation

Emulator Core Distributions (Programs)

libretro Specific

RetroArch, the official libretro distribution with its own frontend

Generic

EmuDeck, a script that sets up a bunch of emulators and installs either ES-DE or Pegasus as the frontend.
RetroDECK, a Flatpak application that uses the ES-DE frontend and comes with several emulators.
Retrobat, a Windows application that uses a fork of Batocera's EmulationStation frontend

Emulator Core Distributions (OS)

libretro Specific

LudoOS, a minimal libretro frontend.

Generic

RetroPie is a gaming dedicated OS inspired by RetroArch for Raspberry Pi SBCs, that comes with a fork of EmulationStation as the graphical frontend
Batocera Linux is a gaming dedicated OS distribution that uses a fork of the old EmulationStation frontend
Lakka is a gaming dedicated OS distribution that uses XcrossMediaBar (XMB) as its frontend
Recalbox [PROPRIETARY]

Backlinks

emulators

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