The ignition is the single most important part of a combustion cannon and requires the most thought.  There are several methods of ignition. (1) flint lantern igniter (2) BBQ igniter  (3)  piezoelectric lighter  (4)  high voltage stun gun.  I'll have pictures up at a later date.

(1) Flint Igniter-This is the most common form of spudgun ignition. It is installed directly into the cleanout plug of the spudgun.  A small spark is created by sharply twisting a brass shaft which is connected to a small metal wheel.  Also a tube containing flint is attached adjacent to the steel wheel.  When the steel wheel turns, it brushes against the flint creating a small spark which is just enough to create the ignition.
Pros
+It works nearly every time.
+Is easy to install
+Require no risk of high voltage electrocution
+Has no contacts to interfere with

Cons
-Flint eventually runs out
-The flint has to periodically be exchanged
-It can get gummed fairly quickly if you don't clean it

(2)BBQ Igniter- This is the second most common form of ignition used.  It uses to contacts which have a high voltage running through them enough so to make a spark jump to form ignition.
Pros
+Works nearly every time
+Is easy to mount 
+Is easy to activate

Cons
-Requires a small chamber to work in
-Is cumbersome when seen mounted
-Is quite expensive
-Uses metal contacts

(3) Piezoelectric Lighter-This is a very good method which works 90% of the time.  This switch is very small and compact (about the size of a penny).  It also uses the spark jumping method.
Pros
+Very compact
+Has a larger spark
+Can use variable contact wired in series

Cons
-Practically none.

The last method is a very dangerous method that is used by the maintainers of the SpudRock page.  This method should only be used by very experienced experts! You should be able to come by it through most major search engines ( Google, Hotbot, Altavista ect...)
If you would like additional info. on these such as pictures, I haven't got them in yet. So drop me an E-mail and I'll send an image or link in which it can be found back.