I'm sure this has been done dozens of times already, but I thought it would be fun to give it a go.
So here's a simple Wolfenstein 3D style ray-caster.
There's no game-play at all, you can just move around.
You can edit the level in the pico8 map editor (make sure it is always enclosed, otherwise you'll get an infinite loop - rays only terminate when they hit a wall).
Update: Cleaned up & optimised code. Added textured ceiling and floor.