I brought out a Genesis to try out some recently purchased games. Ultimate Mortal Kombat 3 is pretty awful looking, whereas Sonic & Knuckles shows that the Genesis is capable of looking good.
As long as I was at it, I grabbed my 2 other copies of S&K, Sonic 2 and a Game Genie. This allowed me to construct... a Sega Tower:


As you can see, it is functional. S&K seems to ignore any other S&Ks plugged into them, and so even with 3 in a row, the Lock-on Technology™ still works with Sonic 2. Put the Game Genie in the middle though, and it takes you to the any-other-game-is-plugged-in mode.
This is only a mini tower, compared to some I've seen. (I could've added a Sega CD, 32X and my other Game Genie, but I wasn't interested in getting them all together.) This one actually works though, while some I've seen even included a cleaning kit into the mix. It wobbles quite a bit too, but didn't glitch up. Some day I'll construct a larger one. Get a few more 32Xs to pile on, a few more Game Genies, and a Power Base Converter and I could erect something quite massive.
Anyone care to try, or seen any impressive photos on the internets?
Edit: Oh, tried out that Game Gear Battery Back. It actually does work, and it does work with the Genesis, but the cord is a little loose.