Most emulators have built-in memory editors, so it's pretty trivial to do in most games, simple things like infinite life or ammo. Especially SNES/GBA era games where many popular titles (esp pokemon) have had their memory mapped entirely.
I figured out Dungeon Keeper's data file at a young age. Really, it was just a tab delimited text file, but it was simple enough to mod. I made it so that every monster almost instantly hit level 10.
One other thing was using an actual hex editor to poke at various game saves, to look at how they formatted their save data. In particular, Castlevania COTM was a little bit interesting because it had a checksum for data, otherwise it would reset into a blank file on startup.