PICO-8 is a “fantasy console” that lets people build and share small games with a full suite of built-in editors. According to the website, “the harsh limitations of PICO-8 are carefully chosen to be fun to work with, encourage small but expressive designs and hopefully to give PICO-8 cartridges their own particular look and feel”. Some of these limitations include a 128×128 display resolution, with a 16-color palette, 128 8×8 sprites, and 4 channels of “chip blerps” for sound and music.
The tools are only available to players who have paid for Lexaloffle’s Voxatron, a voxel-based arena shooter that also comes with its own game engine, but anyone can play PICO-8 carts for free via the online BBS (PICO-8 users can also share carts with one another in a special PNG format). It’s fun just browsing the BBS to check out the various games, demos, and experiments, but if you’re looking for a recommendation, I suggest trying the games by Benjamin Soulé, which really push the console’s limits and show off the wide range of games that can be made with its tools. They’re fun, too!