While not the first hack by a long shot and not even mine but I always loved the idea of how it worked.

There used to be program called Gamehack or something like that. Essentially you would start the game and point this application at said game in RAM, then take note of something like the score being "187" or whatever. Jump into 'Gamehack' and it would search for everything in memory with that value. You would then play for a little bit longer and once the score had changed, you could then jump into game hack and find which of those memory addresses had changed to the new score. Usually you would only have one, you could then change this number to what ever you wanted.

It was such a simple concept but it worked so well. Wouldn't be able to do something like that anymore due to all manner of sandboxing in action. Lost a tool, gained security.

Only other hack was messing with the vehicle stats in Vice City. Ended up with the firetruck that could jump the entire map. Good fun.