(the flash may have fired twice on this exposure, which would explain why you can see a whole balloon and a burst one in the same frame heh)
Made a contact trigger for my SB-800 out of some bits of foil spaced between some cardboard with a hole in it. A wire from each bit of foil led to a terminal on the flash. When the BB went through both bits of foil (and then into the balloon), it mashed the bits together, shorting the circuit and firing the flash :D
D200 at f/5, ISO 100, 4 second exposure. SB-800 camera left, pointing up, triggered by contact trigger (will post a photo of this later / maybe a tutorial on how to make one)
Video tutorials on how to make a contact trigger to shoot things like
www.vimeo.com/865808/ (in nice hi-def).
www.youtube.com/watch?v=GVUxKnNMW44 (in case there's any reason why the above doesn't work).