An ignition coil is the ignition source used in most gasoline engines, and can be adapted for use as a spudgun ignition. Almost any coil will work as long as it's in good condition. Ignition coils will give powerful sparks, which means most circuits will also require a lot of power (the camera flash circuit is an exception).
An ignition coil needs a pulsed current through its primary winding to generate the high voltage, so it cannot simply be connected to a car battery. These are the most common ways to drive an ignition coil:
A relay is connected so that when it activates, it will cut its own current. This will cause it to oscillate, and the resulting pulsed current can be fed to the ignition coil.
There are several ways to set up this circuit depending on the configuration of the relay. The most reliable and powerful design uses a relay with two normally closed contacts, where one is connected in series with the relay solenoid to make it oscillate, and the other contact is connected in series with the ignition coil. The contact that closes first upon return should be connected to the ignition coil, so the contacts should be swapped during testing to see which configuration gives the most consistent spark.
Electronic oscillator circuit
An oscillator circuit, typically made up of a 555 timer circuit and switching transistor, is used to interrupt the current through the coil with a frequency of 5-20 kHz.
The voltage from the storage capacitor of a camera flash is dumped through the ignition coil. See the camera flash article for circuit details.
If the current through the ignition coil is switched on and off manually, the coil will produce a spark every time the current is shut off. A crude way to create many sparks in rapid succession consists of using the surface of a file, when one wire is connected to the file and the other raked over its surface, the current will be interrupted many times in rapid succession.
An ignition coil can be driven by mains power, by connecting it in series with a light dimmer and a motor start capacitor. The dimmer should be set at about 50% for maximum voltage. The dimmer will chop the mains power and discharge the capacitor through the coil. This circuit is dangerous since it carries mains voltage on all parts, and should NOT be used for spudgun ignition.