RetroArch

You can install retroarch with some cores (actual emulators) by putting something like this in your /etc/nixos/configuration.nix:

environment.systemPackages = with pkgs; [
  (retroarch.override {
    cores = with libretro; [
      genesis-plus-gx
      snes9x
      beetle-psx-hw
    ];
  })
];

The cores are defined here:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/emulators/retroarch/cores.nix#L61

Gotchas

Not all the graphics settings are in the same place. You'll want to look under the top-level Settings (in the left column under Main Menu) and also under Main Menu -> Quick Menu -> Options

Bugs

Input Reset

beetle-psx-hw input (by keyboard anyway) only works if you go to Settings -> Input -> Port 1 Controls -> Device Type and set it to PlayStation Controller every time you run the emulator.

Missing Icons

No icons show in the GUI, including the mouse cursor. You need to go to Online Updater > Update Assets to download the icons. You can navigate the GUI with only the keyboard. Press enter to open sections/pages, backspace to go back out, arrow keys to choose between options (left/right change some values).