How many addons CAN you add to a Sega?

I worked as a video game store manager back in the day when these add-ons were the latest thing - check out how many they managed to stack together! Really cracked me up - so far I can identify:
Sega CD
Sega 32X
Game Genie
Sonic & Knuckles
Sonic 3

but there's a few the light is JUST wrong on that I can't make out - post 'em in the comments thread if you know them!



Original blog post here:
http://www.joystiq.com/2007/01/23/the-leaning-tower-of-sega/