PICO-8 is a 
Lua based fantasy console for programming, sharing and playing tiny games and other computer programs. The app boots to a shell prompt with easy access to scores of game building tools.
When you turn it on, the machine greets you with a commandline, a suite of cartridge creation tools, and an online cartridge browser called SPLORE.
Specifications
The harsh limitations of PICO-8 are carefully chosen to be fun to work with, to encourage small but expressive designs, and to give cartridges made with PICO-8 their own particular look and feel.
Creative Tools
PICO-8 has tools for editing code, music, sound, sprites, maps built right into the console. Create a whole game or program in one sitting without needing to leave the cosy development environment!
Shareable Cartridges
PICO-8 cartridges can be saved in a special .png format and sent directly to other users, shared with anyone via a web cart player, or exported to stand-alone HTML5, Windows, Mac and Linux apps.
Any cartridge can be opened again in PICO-8, letting you peek inside to modify or study the code, graphics and sound.
Explore the Cartverse
PICO-8 comes with a built-in cartridge browser called SPLORE, for searching and favouriting carts from the online collection.
Community Resources
PICO-8 has a friendly community of users collaborating, sharing knowledge and creating tools, snippets and tutorials. Check out what people are up to on the forums, twitter, and other places.
Just Add Hardware
PICO-8 is tiny to download, easy to install, and will run on almost anything! To use PICO-8, you'll need either Windows, a Mac, Linux (i386 / amd64), or a Raspbery Pi with ~700MHz CPU. Turn your old unused netbooks or microcomputers into PICO-8s!
https://www.pico-8-edu.com/
There is also a freely available web version of PICO-8 that has no exporters or permanent storage.
PICO-8 vs Voxatron?
You can read about the intertwined history of the two projects in Issue #1 of the PICO-8 zine. They are now two separate projects but both follow the same fantasy console format, and also share some components. The PICO-8 display (128x128) is one slice of Voxatron's display, and they both include the same audio mixer and tools. Voxatron 0.4 will be able to load and play PICO-8 cartridges.
New Zealand
Kezxo added PICO-8 as alternative to LÖVE and Godot Engine
Comments and Reviews
A great tool for those learning game development, or who are tired of extraneous workflows. As far as fantasy consoles go, PICO-8 is the most streamlined and cohesive.
Arguably the grandfather of all current fantasy consoles and the best at it.
Pico-8 is fun to develop in, on almost any platform you want (PC, RaspPi, Linux, Mac). The community surrounding Pico8 is excellent: very helpful and creative.