My uncle has a Pc-ez bake oven that goes in there. It can fit mini waffles or pancakes and they taste great. Don't know where he got it but I'm sure there are more out there. I have seen some people make cup holders out of them. The coolest thing I saw was a when someone made a stand for his tablet out of one and he could control the computer with his tablet by the click of a button.
Fan controllers, optical drives, USB/SD card readers, 5.25" water cooling reservoirs, the drawer mentioned above, LED controls, hot swap HDD/SSD bays, fans, temperature readouts.
Thats all that I'v seen them used for.