For those unable/too cheap to buy a cracker, i've whipped up some designs for a cracker made from plumbing parts. The thread is over at Aus DD:
http://www.bluelight.ru/vb/showthread.php?t=386002
For those worried about string not being sturdy enough to hold the nail down in design 1, stop freaking out. Ive done the maths and its solid as a rock. I'm an engineering student, i know how to build shit
While not recommended, some people have had success using design 1 without a balloon, design 2 is untested ATM, i only designed & built it yesterday (thank you, ritalin

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Edit:
for those wondering, the reason the cracker/dispenser gets cold is the fact that as a gas expands it gets gets colder. The more it expands the colder it gets. Now a whipped cream dispenser has a volume of 1-2 litres, the average party balloon (30cm diameter say) has a volume of about 14 litres, so the nitrous expands much more using a cracker and balloon than using a whipped cream dispenser.
And that is why a balloon/cracker combo gets colder than a whipped cream dispenser