http://imageshack.us/photo/my-images/21 ... 100357.jpg

The part on top of the throttle body that has a wire plug connected to it, and usually has a black knob on the end of it (mines broken off). What is it and what does it do?

Cheers

Idle Control Solenoid or Air bypass system.
Quoted from tech doc
The Idle Speed Controller Valve, or otherwise known as the Idle Air Bypass System, is responsible for maintaining a smooth, and consistent idle, under different Engine Loads, and temperatures.
It does this by allowing a certain amount of air, to bypass the Throttle Body Butterfly, thereby increasing the amount of air entering the engine, under certain conditions, thereby increasing Engine RPM.
The Idle speed control valve is controlled by the EEC-V Engine Management system, and is a spring loaded stepped motor, that opens a small valve, that allows air to flow.

Could the fact that the black knob on mine is brokn off cause any issues? Also where the knob was it looks like the previous owner has stuffed a match stick into the shaft lol.

the black knob is actually a filter. shouldnt be blocked off with a matchstick.