This semester I'm starting a project as an independent study under Mark Brehob at University of Michigan. I'm trying to build a Super Nintendo cartridge with an SD card slot. The idea is that you can copy ROM images to an SD card, and then you can play them on your real Super Nintendo.
Around the internet, I've seen similar projects before. The general hobby solution to this problem is to use 5v NOR (parallel) flash chip wired as drop-in replacement for the mask ROM chips on an existing Super Nintendo cartridge. Basically, you write some ROM data to the flash chip, and then solder it up to the cart in place of the old ROM chip. This isn't very sustainable, as parallel NOR flash is being phased out in favor of serial NAND, and for the most part the 5v parts have already been phased out (digikey doens't even sell them anymore). This solution sucks anyway, because carts can't easily support multiple games. Also, there isn't an easy way to reprogram the cart without more custom hardware.
There are some more advanced commercial solutions. During the heyday of the SNES, there was a number of "copier" units. As far as I know, the only one still in production is the Super Flash Card from ToToTek. It is a flash based solution, and needs to be programmed over the parallel port (yuck!) using a seperate programming device. that plugs into the cart. The programming device has a USB port, but don't be fooled. It's only for power. This product supports one of the special chips found in some SNES carts, the DSP-1, using some sort of pass-through method (you plug in a working dsp-1 game with a t-connector). I don't own one.
I see the Super Flash Card as an extension of the hobbyist carts. It takes things one step further, using a Xilinx CPLD (or FPGA?) for memory mapping, so you can load multiple games on one cart. The cart includes battery-backed SRAM and the mapper can deal with the different types of SRAM saving (the size and memory address of sram is different across games). Unfortunately, you can only hold saves from one game on the cart at a time.
I can understand why the product made these design decisions, as it simplifies the product drastically. My design will require a more complex FPGA/CPLD core. It will have to use SDRAM to store the game after loading it from the SD card so that I can meet the bus timing requirements of the SNES. Also, because I am using modern 3v parts, I will most likely need level converters to deal with the 5v input from the SNES.
It should be interesting, I think.