What is

a box that lets you play arcade games (ie the original boards) on a tv set, with controllers taken from whatever system you can get thats suitable

usually a custom made pcb and power supply
Sounds interesting and very useful as I have an arcade cabinet just sitting in my basement, where would i get the plans to make one?
A supergun is usually a homemade system designed to play arcade game boards on a tv. It is basicly a power supply, a RGB>NTSC/PAL converter, and a wiring harness for controls. There are a few commercial systems I know of, but it is fairly easy to build one if you have some basic electronic exp. Let me know if you need more info/links on this.
Is your cabinent "jamma"?

Actually, you can use a pc power supply. The only part that is custom is the video conv. You can buy one from massystems.com or gwtrading.com for around $150. I recommend http://www.jrok.com/hardware/RGB.html. As for controls, just put in an order with happcontrols.com.

Please check out arcadecontrols.com for more info.

one of the SGs I built. since TVs have RGB connection in germany, there was no need for the RGB converter.

it contains everything needed.. TV cable, PSU connector, 2 ports with neogeo pinout, coin and testmode button, power LED, in a hand-cut case with jamma connector at the bottom.

I use a modified saturn pad with it.

as you can see, it's not that much stuff really, just alot of soldering involved.