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.
For info on flash memory chips, you can look at:
(I'm not sure how much longer this URL will be valid, they appear to be moving it)
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.
Hope this helps.