[coldpie]

...which eventually leads to figuring out how to write arbitrary values to RAM and jump the CPU to execute those values. Witness the Pokemon Yellow Total Control Hack:

http://tasvideos.org/3767S.html

These are becoming more popular. There's similar ones for later Pokemon games as well as more impressive titles like Super Mario World for the SNES:

http://tasvideos.org/4156S.html