Where can you play 3D monster maze online for free

Try going here:

http://www.zx81stuff.org.uk/zx81/emulate.php?tzx=0%2F3DMonsterMaze(NGS).tzx.zip%400&title=3D+Monster+Maze

However, you might need to install Java in order to play it. This online version is the actual version that is played on the ZX81 computer. But to be honest, I don't think this version is illegal to play and getting into a game is confusing, especially
if you are trying to figure out what the keys do if you are using a plain keyboard. But the game when load on its own when starting up. There is also a pause feature if you click on the screen or click outside of the game.

I hope this answer will help you.