Raph has compiled a great page of hacks for the venerable old NES. Some of them – like a remote reset button, stereo sound output and blue power LED – are fairly straightforward. Others, such as his programmable cartridge mod, are pure Ninja.
The most impressive hack of all is the modified NES that runs the arcade version of Super Mario Bros. This one required a bit of palette remapping and some hardware tweaking. It works because the old Unisystem VS arcade motherboard is basically the equivalent of two NES home machines on a single board, with the game stored in on-board ROM.Raph's NES Stuff Page