Two games published by Activision on the C64, Hacker and Hacker II. Both are meant to simulate remote terminal 'hacking'. The first game originally began as a robot driving through underground tunnels, delivering mail. Of course that would not make a very fun game. So you are not told what is happening at all. No clues, No instructions apart from a little bit of background.
Completing this game requires a lot of patience (less now, what with fast emulators of course, but back then this game took a large chunk out of your time), educated guesses, and a bit of trial and error.
Of course even with the large amount of patience requires, the game needs less than what it is attempting to simulate.
This game is a pure lesson in learning. Play it once if only to see what learning is like.
It has now been converted to a windows game on the 'Activision C64 15 Pack'.