I sort of inspected a few of my games that used a password save system and noticed patterns that let me modify them. For instance, in Mega Man 2 I collected E Tanks only(did nothing else) and noticed what happened to the password. Collecting the maximum amount and noticing the password allowed me to create a password where at any point I could refill my character back to maximum E Tanks. I was able to do the same to a few other games as well and it was always fun...
I never reversed engineered passwords, but I did like to enter random ones in to see what, if anything, would happen. Similar to the Game Genie post from a while back.
I kinda wish I had done some trial and error like that now. I wonder if there are any more game specific guides out there for old games so you can start with a bajillion lives or whatever.
I did notice that punch out seemed to have entirely random passwords. I had a password for Mr. Dream and then lost to him, and the password it gave me was entirely different. Theoretically I should have just had one more loss on my record.
The password system could be very sequential but then they use some kind of one off simple encryption / scramble to make it seem randomish.