Guns: Shotgun Lights
I suppose most of the gadgets people hang off their home defense shotguns are not strictly necessary. The chances are slim that you'll ever need to use that fancy sidesaddle or Speedfeed stock in a dangerous situation. One accessory that I don't consider optional in a housegun is a good weaponlight.
The shotgun poses special problems to fielding a good lighting solution, however. It's definitely the consensus on the Web that the recoil of full-power 12 gauge buckshot and slugs is enough to knock out most casual lights after sustained use. And for my preferred scattergun, the Remington 870, you must always have your support arm on the foreend, ready to slide in another shell. All of this means that "lighting the cheap seats," as Tam puts it, is a bit more difficult.
The ne plus ultra of the shotgun weaponlight game is the SureFire replacement foreend. Costing $250-$300 (almost as much as the average pump shotgun), it's not an impulse buy, but it's certainly one of the nicer options. I'm a SureFire fanboy, even though objectively other companies like PentagonLight and Streamlight make good stuff.
I'm considering trying out one of the more inexpensive mounts - perhaps using a a bracket and/or rail attachment and a standard TLR-1 weaponlight (originally intended for use on a handgun with an M3 rail). They often look kind of, well, gnarly, though: