Seeing this one on the list makes me sad I can only vote once - I honestly can't say how many hours I burned on this game on my Atari ST. The game itself was free, but to get past the first 10 or so levels you had to buy "the dongle": a book of code combinations, printed in low-contrast colours to make photocopying virtually impossible. 35 years later I still have the book, and I would dearly love to play Oxyd again.