I think that somewhere in the Net MUST BE information about replacing rom in a cartdrige with eeproms and programming them to play a downloaded rom into the console... if somebody knows info about, I'm really interested (SNES, MEGA DRIVE basically ^^)
The SMSReader site is very nice, in that it walks you through constructing the unit. More importantly, it does a pretty good job of explaining what all the components are for. Unfortunately, there's no Genesis adapter for it, though I remember hearing at one point that Mike G was working on one. If you're interested in coming up with a Genesis or SNES design yourself, I suggest looking through the Genesis hardware internals doc on Charles MacDonald's site for a mostly complete Genesis cartridge pinout with signal descriptions, and the SNESKart document for info on the SNES cartridge interface.
Toshiba, Micron, and Samsung also make DRAM, which can't interface directly with SNES/Genesis, and SRAM, which is somewhat easier to interface than flash, but substantially more expensive.
I'm working on designing a flash-based Genesis cartridge, but I don't know when that will really be done, nor have I decided whether or not I'm going to release the design files to build it (I probably will, but I haven't really made the decision yet and it's been bugging me a bit). If you have any questions, I'll try to answer them, but I'm not to the point where I can actually build a prototype yet (due to lack of equipment, which I plan to get soon), so I don't have any personally tested info about interfacing with Genesis.